Add support for golang.org/x remote libs.

Although golang.org/x supports the `go get` <meta> protocol, pants does
not have a `Fetcher` that supports that protocol yet, so just use the
standard `ArchiveFetcher` with a regex mapping that redirects directly
to github (this appears to be the consistent pattern of th golang.org/x
<meta> re-directs).

Add a test to cover this new default fetcher configuration.

Testing Done:
Manually tested this in a custom Medium pants.ini setup:
```ini
[fetchers]
mapping: {
    # Builtin fetchers:
    r'^bitbucket.org/.*$': 'ArchiveFetcher',
    r'^github.com/.*$':'ArchiveFetcher',
    r'^gopkg.in/.*$':'GopkgInFetcher',

    # Our workaround custom fetcher until
    # pantsbuild#2378 or
    # pantsbuild#2379 is resolved.
    r'^golang.org/x/.*$':'ArchiveFetcher',
  }

[archive-fetcher]
matchers: {
    # Builtin matchers:
    r'bitbucket.org/(?P<user>[^/]+)/(?P<repo>[^/]+)':
      ('https://bitbucket.org/\g<user>/\g<repo>/get/{rev}.tar.gz', 'tip', 1),
    r'github.com/(?P<user>[^/]+)/(?P<repo>[^/]+)':
      ('https://github.com/\g<user>/\g<repo>/archive/{rev}.tar.gz', 'master', 1),

    # Our workaround custom fetcher until
    # pantsbuild#2378 or
    # pantsbuild#2379 is resolved.
    r'golang.org/x/(?P<user>[^/]+)':
      ('https://github.com/golang/\g<user>/archive/{rev}.tar.gz', 'master', 1),
  }
```

CI went green here:
  https://travis-ci.org/pantsbuild/pants/builds/85667224

Bugs closed: 2378, 2379, 2380

Reviewed at https://rbcommons.com/s/twitter/r/2976/

contrib/go/src/python/pants/contrib/go/subsystems/fetchers.py

@@ -280,14 +280,18 @@ def rev(self, rev):
   options_scope = 'archive-fetcher'
 
   _DEFAULT_MATCHERS = {
-    r'bitbucket.org/(?P<user>[^/]+)/(?P<repo>[^/]+)':
+    r'bitbucket\.org/(?P<user>[^/]+)/(?P<repo>[^/]+)':
       UrlInfo(url_format='https://bitbucket.org/\g<user>/\g<repo>/get/{rev}.tar.gz',
               default_rev='tip',
               strip_level=1),
-    r'github.com/(?P<user>[^/]+)/(?P<repo>[^/]+)':
+    r'github\.com/(?P<user>[^/]+)/(?P<repo>[^/]+)':
       UrlInfo(url_format='https://github.com/\g<user>/\g<repo>/archive/{rev}.tar.gz',
               default_rev='master',
               strip_level=1),
+    r'golang\.org/x/(?P<repo>[^/]+)':
+      UrlInfo(url_format='https://github.com/golang/\g<repo>/archive/{rev}.tar.gz',
+              default_rev='master',
+              strip_level=1),
   }
 
   @classmethod
@@ -569,3 +573,4 @@ def _select_highest_compatible(self, candidates, raw_rev):
 Fetchers.advertise(ArchiveFetcher, namespace='')
 Fetchers._register_default(r'^bitbucket\.org/.*$', ArchiveFetcher)
 Fetchers._register_default(r'^github\.com/.*$', ArchiveFetcher)
+Fetchers._register_default(r'^golang\.org/x/.*$', ArchiveFetcher)

contrib/go/tests/python/pants_test/contrib/go/subsystems/test_fetchers.py

@@ -13,7 +13,7 @@
 from pants.util.contextutil import temporary_dir
 from pants_test.subsystem.subsystem_util import subsystem_instance
 
-from pants.contrib.go.subsystems.fetchers import Fetchers, GopkgInFetcher
+from pants.contrib.go.subsystems.fetchers import ArchiveFetcher, Fetchers, GopkgInFetcher
 
 
 class FetchersTest(unittest.TestCase):
@@ -51,10 +51,52 @@ def test_default_github(self):
     self.check_default('github.com/docker/docker/daemon/events',
                        expected_root='github.com/docker/docker')
 
+  def test_default_golang(self):
+    self.check_default('golang.org/x/oauth2',
+                       expected_root='golang.org/x/oauth2')
+    self.check_default('golang.org/x/net/context',
+                       expected_root='golang.org/x/net')
+
   def test_default_gopkg(self):
     self.check_default('gopkg.in/check.v1', expected_root='gopkg.in/check.v1')
 
 
+class GolangOrgFetcherTest(unittest.TestCase):
+
+  def do_fetch(self, import_path, expected_url, rev=None):
+    with subsystem_instance(ArchiveFetcher) as fetcher:
+      fetcher._fetch = mock.MagicMock(spec=fetcher._fetch)
+      with temporary_dir() as dest:
+        fetcher.fetch(import_path, dest, rev=rev)
+
+      # For some reason using `assert_called_once_with` like so:
+      #   fetcher._fetch.assert_called_once_with(expected_url)
+      #
+      # Yields this error:
+      # E       AssertionError:
+      #           Expected call: mock(u'https://github.com/golang/oauth2/archive/master.tar.gz')
+      # E       Actual call: mock(u'https://github.com/golang/oauth2/archive/master.tar.gz')
+      # E       too many positional arguments
+      #
+      # So we manually check called once with in 2 steps.
+      self.assertEqual(1, fetcher._fetch.call_count)
+      self.assertEqual(mock.call(expected_url), fetcher._fetch.call_args)
+
+  def test_fetch(self):
+    self.do_fetch(import_path='golang.org/x/oauth2',
+                  expected_url='https://github.com/golang/oauth2/archive/master.tar.gz')
+    self.do_fetch(import_path='golang.org/x/net/context',
+                  expected_url='https://github.com/golang/net/archive/master.tar.gz')
+
+  def test_fetch_rev(self):
+    self.do_fetch(import_path='golang.org/x/oauth2',
+                  rev='abc123',
+                  expected_url='https://github.com/golang/oauth2/archive/abc123.tar.gz')
+    self.do_fetch(import_path='golang.org/x/net/context',
+                  rev='def456',
+                  expected_url='https://github.com/golang/net/archive/def456.tar.gz')
+
+
 class GopkgInFetcherTest(unittest.TestCase):
 
   def do_fetch(self, import_path, version_override=None, github_api_responses=None,