superproject: print messages if the manifest has superproject tag.

1) If the manifest has superproject tag (git_master, etc), then
   display error/warning messages (as it is doing today)
2) If the manifest doesn't have superproject tag (nest, chromeos
   manifests), then don't display any error/warning messages about
   superrproject (behave as though user has specified
   --no-use-superproject).
3) Print error/warning messages if --use-superproject passed as
   argument to repo sync.
4) No change in behavior for the repo init command.

git_superproject.py:
+ Fixed typo in _WriteManifestFile method name
+ Superproject accepts print_message  as an argument and it defaults
  to True. All messages that are printed to stderr are controlled by
  this flag. If it is True, then messages get printed.
+ Added PrintMessages function which return true if either
  --use-superproject is specified on the command line or if the
  manifest has a superproject tag.

sync.py:
+ Displays the warning message if PrintMessgages are enabled and
  passes that as argument to superproject object.
+ Added 'hassuperprojecttag' trace2 log entry for analysis. We can
  find users/branches that are using superproject, but the manifest is
  missing the superproject tag.

Tested:
$ ./run_tests

+ Verified printing of messages with and without superproject tag, with
  with --use-superproject option.

+ aosp-master
  $ repo_dev init --use-superproject -u https://android.googlesource.com/platform/manifest
  $ repo_dev sync

+ A manifest without superproject tag.
  $ repo_dev init -m $(pwd)/manifest_7482982.xml
  $ repo_dev sync -n -c -j32 -m $(pwd)/manifest_7482982.xml

Bug: [google internal] b/196411099
Change-Id: I92166dcad15a4129fab82edcf869e7c8db3efd4b
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/314982
Reviewed-by: Xin Li <delphij@google.com>
Tested-by: Raman Tenneti <rtenneti@google.com>

tests/test_git_superproject.py

@@ -203,7 +203,7 @@ class SuperprojectTestCase(unittest.TestCase):
     project.SetRevisionId('ABCDEF')
     # Create temporary directory so that it can write the file.
     os.mkdir(self._superproject._superproject_path)
-    manifest_path = self._superproject._WriteManfiestFile()
+    manifest_path = self._superproject._WriteManifestFile()
     self.assertIsNotNone(manifest_path)
     with open(manifest_path, 'r') as fp:
       manifest_xml_data = fp.read()