Remove group task and register zinc_compile directly

GroupTask was only used by one task, and there was only one task left in the group.

This change has a side deffect of moving some path names used in the buildcache.

Testing Done:
CI is green at https://travis-ci.org/pantsbuild/pants/builds/95272210

Bugs closed: 2669

Reviewed at https://rbcommons.com/s/twitter/r/3215/

src/python/pants/backend/core/tasks/BUILD

@@ -49,7 +49,6 @@ target(
     ':dependees',
     ':filemap',
     ':filter',
-    ':group_task',
     ':list_owners',
     ':listtargets',
     ':markdown_to_html',
@@ -150,19 +149,6 @@ python_library(
   ],
 )
 
-python_library(
-  name = 'group_task',
-  sources = ['group_task.py'],
-  dependencies = [
-    '3rdparty/python:six',
-    'src/python/pants/base:workunit',
-    'src/python/pants/build_graph',
-    'src/python/pants/goal',
-    'src/python/pants/option',
-    'src/python/pants/task',
-  ],
-)
-
 python_library(
   name = 'listtargets',
   sources = ['listtargets.py'],
@@ -230,7 +216,6 @@ python_library(
   name = 'reflect',
   sources = ['reflect.py'],
   dependencies = [
-    ':group_task',
     '3rdparty/python:docutils',
     '3rdparty/python:six',
     'src/python/pants/base:build_environment',