manifest_xml: Add Load and Unload methods

- do not call the internal method from subcmds/sync.py.
- use the correct default groups for submanifests.
- only sync the superproject when we are told to.

Change-Id: I81e4025058f1ee564732b9e17aecc522f6b5f626
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/334639
Reviewed-by: Mike Frysinger <vapier@google.com>
Reviewed-by: Raman Tenneti <rtenneti@google.com>
Tested-by: LaMont Jones <lamontjones@google.com>

project.py

@@ -3491,6 +3491,8 @@ class ManifestProject(MetaProject):
     """
     assert _kwargs_only == (), 'Sync only accepts keyword arguments.'
 
+    groups = groups or 'default'
+    platform = platform or 'auto'
     git_event_log = git_event_log or EventLog()
     if outer_manifest and self.manifest.is_submanifest:
       # In a multi-manifest checkout, use the outer manifest unless we are told
@@ -3783,19 +3785,18 @@ class ManifestProject(MetaProject):
         )
 
     # Lastly, clone the superproject(s).
-    if outer_manifest and not self.manifest.is_submanifest:
-      for m in self.manifest.all_manifests:
-        sync_result = Superproject(
-            m, m.repodir, git_event_log, quiet=not verbose).Sync()
-        if not sync_result.success:
-          print(f'warning: git update of superproject for {m.path_prefix} failed, '
-                'repo sync will not '
-                'use superproject to fetch source; while this error is not fatal, '
-                'and you can continue to run repo sync, please run repo init with '
-                'the --no-use-superproject option to stop seeing this warning',
-                file=sys.stderr)
-          if sync_result.fatal and use_superproject is not None:
-            return False
+    if self.manifest.manifestProject.use_superproject:
+      sync_result = Superproject(
+          self.manifest, self.manifest.repodir, git_event_log, quiet=not verbose).Sync()
+      if not sync_result.success:
+        print('warning: git update of superproject for '
+              f'{self.manifest.path_prefix} failed, repo sync will not use '
+              'superproject to fetch source; while this error is not fatal, '
+              'and you can continue to run repo sync, please run repo init '
+              'with the --no-use-superproject option to stop seeing this '
+              'warning', file=sys.stderr)
+        if sync_result.fatal and use_superproject is not None:
+          return False
 
     return True