Add multi-manifest support with <submanifest> element

To be addressed in another change:
 - a partial `repo sync` (with a list of projects/paths to sync)
   requires `--this-tree-only`.

Change-Id: I6c7400bf001540e9d7694fa70934f8f204cb5f57
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/322657
Tested-by: LaMont Jones <lamontjones@google.com>
Reviewed-by: Mike Frysinger <vapier@google.com>

git_superproject.py

@@ -92,7 +92,8 @@ class Superproject(object):
     self._branch = manifest.branch
     self._repodir = os.path.abspath(repodir)
     self._superproject_dir = superproject_dir
-    self._superproject_path = os.path.join(self._repodir, superproject_dir)
+    self._superproject_path = manifest.SubmanifestInfoDir(manifest.path_prefix,
+                                                          superproject_dir)
     self._manifest_path = os.path.join(self._superproject_path,
                                        _SUPERPROJECT_MANIFEST_NAME)
     git_name = ''