Move task.py out of backend/core.

It's a core pants concept and not part of what we
usually mean by "backend".  This is a step one of
sorting out the mess in backend/core.

Points all pants code to the new location of task.py,
which is a new top-level 'task' package. A future change
will move ConsoleTask and other task-related stuff to there.

There are forwarding aliases in the original task.py, with a
deprecation warning, so external plugins shouldn't break.

Testing Done:
CI passes: https://travis-ci.org/pantsbuild/pants/builds/91325801

Reviewed at https://rbcommons.com/s/twitter/r/3130/

contrib/cpp/src/python/pants/contrib/cpp/tasks/BUILD

@@ -14,9 +14,9 @@ python_library(
   dependencies=[
     'contrib/cpp/src/python/pants/contrib/cpp/toolchain:toolchain',
     'contrib/cpp/src/python/pants/contrib/cpp/targets:targets',
-    'src/python/pants/backend/core/tasks:common',
     'src/python/pants/base:exceptions',
     'src/python/pants/base:workunit',
+    'src/python/pants/task',
     'src/python/pants/util:dirutil',
   ],
 )

contrib/cpp/src/python/pants/contrib/cpp/tasks/cpp_task.py

@@ -7,8 +7,8 @@
 
 import subprocess
 
-from pants.backend.core.tasks.task import Task
 from pants.base.exceptions import TaskError
+from pants.task.task import Task
 
 from pants.contrib.cpp.targets.cpp_binary import CppBinary
 from pants.contrib.cpp.targets.cpp_library import CppLibrary

contrib/go/src/python/pants/contrib/go/tasks/BUILD

@@ -75,8 +75,8 @@ python_library(
     '3rdparty/python/twitter/commons:twitter.common.collections',
     '3rdparty/python:ansicolors',
     'contrib/go/src/python/pants/contrib/go/tasks:go_workspace_task',
-    'src/python/pants/backend/core/tasks:task',
     'src/python/pants/base:exceptions',
+    'src/python/pants/task',
   ]
 )
 
@@ -100,7 +100,7 @@ python_library(
     'contrib/go/src/python/pants/contrib/go/targets:go_library',
     'contrib/go/src/python/pants/contrib/go/targets:go_local_source',
     'contrib/go/src/python/pants/contrib/go/targets:go_remote_library',
-    'src/python/pants/backend/core/tasks:task',
+    'src/python/pants/task',
     'src/python/pants/util:memo',
   ]
 )

contrib/go/src/python/pants/contrib/go/tasks/go_go.py

@@ -10,8 +10,8 @@
 from abc import abstractmethod
 
 from colors import green, red, yellow
-from pants.backend.core.tasks.task import QuietTaskMixin
 from pants.base.exceptions import TaskError
+from pants.task.task import QuietTaskMixin
 from twitter.common.collections import OrderedSet
 
 from pants.contrib.go.tasks.go_workspace_task import GoWorkspaceTask

contrib/go/src/python/pants/contrib/go/tasks/go_task.py

@@ -5,7 +5,7 @@
 from __future__ import (absolute_import, division, generators, nested_scopes, print_function,
                         unicode_literals, with_statement)
 
-from pants.backend.core.tasks.task import Task
+from pants.task.task import Task
 from pants.util.memo import memoized_property
 
 from pants.contrib.go.subsystems.go_distribution import GoDistribution

contrib/node/src/python/pants/contrib/node/tasks/BUILD

@@ -35,8 +35,8 @@ python_library(
     'contrib/node/src/python/pants/contrib/node/targets:node_remote_module',
     'contrib/node/src/python/pants/contrib/node/targets:npm_package',
     'contrib/node/src/python/pants/contrib/node/targets:npm_test',
-    'src/python/pants/backend/core/tasks:task',
     'src/python/pants/base:workunit',
+    'src/python/pants/task',
     'src/python/pants/util:memo',
   ]
 )

contrib/node/src/python/pants/contrib/node/tasks/node_task.py

@@ -5,8 +5,8 @@
 from __future__ import (absolute_import, division, generators, nested_scopes, print_function,
                         unicode_literals, with_statement)
 
-from pants.backend.core.tasks.task import Task
 from pants.base.workunit import WorkUnit, WorkUnitLabel
+from pants.task.task import Task
 from pants.util.memo import memoized_property
 
 from pants.contrib.node.subsystems.node_distribution import NodeDistribution

pants-plugins/src/python/internal_backend/sitegen/tasks/BUILD

@@ -14,7 +14,7 @@ python_library(
   dependencies=[
     '3rdparty/python:six',
     'pants-plugins/3rdparty/python:beautifulsoup4',
-    'src/python/pants/backend/core/tasks:task',
     'src/python/pants/base:exceptions',
+    'src/python/pants/task',
   ]
 )

pants-plugins/src/python/internal_backend/sitegen/tasks/sitegen.py

@@ -15,8 +15,8 @@
 import pystache
 from six.moves import range
 
-from pants.backend.core.tasks.task import Task
 from pants.base.exceptions import TaskError
+from pants.task.task import Task
 
 
 """Static Site Generator for the Pants Build documentation site.

src/python/pants/backend/android/tasks/BUILD

@@ -20,8 +20,8 @@ python_library(
   name = 'android_task',
   sources = ['android_task.py'],
   dependencies = [
-    'src/python/pants/backend/core/tasks:common',
     'src/python/pants/backend/android:android_distribution',
+    'src/python/pants/task',
   ],
 )
 
@@ -82,10 +82,10 @@ python_library(
     'src/python/pants/backend/android:android_config_util',
     'src/python/pants/backend/android:keystore_resolver',
     'src/python/pants/backend/android/targets:android',
-    'src/python/pants/backend/core/tasks:common',
     'src/python/pants/base:exceptions',
     'src/python/pants/base:workunit',
     'src/python/pants/java/distribution:distribution',
+    'src/python/pants/task',
     'src/python/pants/util:dirutil',
   ],
 )
@@ -95,14 +95,14 @@ python_library(
   sources = ['unpack_libraries.py'],
   dependencies = [
     'src/python/pants/backend/android/targets:android',
-    'src/python/pants/backend/core/tasks:task',
     'src/python/pants/backend/jvm/targets:jvm',
     'src/python/pants/backend/jvm/tasks:jar_import_products',
     'src/python/pants/backend/jvm:jar_dependency_utils',
     'src/python/pants/base:build_environment',
     'src/python/pants/base:fingerprint_strategy',
     'src/python/pants/build_graph',
     'src/python/pants/fs',
+    'src/python/pants/task',
   ],
 )
 

src/python/pants/backend/android/tasks/android_task.py

@@ -6,7 +6,7 @@
                         unicode_literals, with_statement)
 
 from pants.backend.android.distribution.android_distribution import AndroidDistribution
-from pants.backend.core.tasks.task import Task
+from pants.task.task import Task
 
 
 class AndroidTask(Task):

src/python/pants/backend/android/tasks/sign_apk.py

@@ -12,10 +12,10 @@
 from pants.backend.android.android_config_util import AndroidConfigUtil
 from pants.backend.android.keystore.keystore_resolver import KeystoreResolver
 from pants.backend.android.targets.android_binary import AndroidBinary
-from pants.backend.core.tasks.task import Task
 from pants.base.exceptions import TaskError
 from pants.base.workunit import WorkUnitLabel
 from pants.java.distribution.distribution import DistributionLocator
+from pants.task.task import Task
 from pants.util.dirutil import safe_mkdir
 
 

src/python/pants/backend/android/tasks/unpack_libraries.py

@@ -10,7 +10,6 @@
 
 from pants.backend.android.targets.android_library import AndroidLibrary
 from pants.backend.android.targets.android_resources import AndroidResources
-from pants.backend.core.tasks.task import Task
 from pants.backend.jvm.jar_dependency_utils import M2Coordinate
 from pants.backend.jvm.targets.jar_dependency import JarDependency
 from pants.backend.jvm.targets.jar_library import JarLibrary
@@ -19,6 +18,7 @@
 from pants.base.fingerprint_strategy import DefaultFingerprintStrategy
 from pants.build_graph.address import Address
 from pants.fs.archive import ZIP
+from pants.task.task import Task
 
 
 class AndroidLibraryFingerprintStrategy(DefaultFingerprintStrategy):

src/python/pants/backend/codegen/tasks/BUILD

@@ -18,8 +18,8 @@ python_library(
   name = 'common',
   sources = ['__init__.py'],
   dependencies = [
-    'src/python/pants/backend/core/tasks:task',
     'src/python/pants/base:exceptions',
+    'src/python/pants/task',
   ]
 )
 
@@ -115,11 +115,11 @@ python_library(
   sources = ['simple_codegen_task.py'],
   dependencies = [
     '3rdparty/python/twitter/commons:twitter.common.collections',
-    'src/python/pants/backend/core/tasks:task',
     'src/python/pants/base:build_environment',
     'src/python/pants/base:exceptions',
     'src/python/pants/base:workunit',
     'src/python/pants/build_graph',
+    'src/python/pants/task',
     'src/python/pants/util:dirutil',
   ],
 )

src/python/pants/backend/codegen/tasks/simple_codegen_task.py

@@ -12,12 +12,12 @@
 
 from twitter.common.collections import OrderedSet
 
-from pants.backend.core.tasks.task import Task
 from pants.base.build_environment import get_buildroot
 from pants.base.exceptions import TaskError
 from pants.base.workunit import WorkUnitLabel
 from pants.build_graph.address import Address
 from pants.build_graph.address_lookup_error import AddressLookupError
+from pants.task.task import Task
 from pants.util.dirutil import fast_relpath, safe_delete, safe_walk