style: use import git_superproject everywhere

Align with Google style guides and `import git_superproject` as a module
rather than import symbols out of it.

Change-Id: I939f1123d5c6f79b56c244a5ffdf5bcfa136b2db
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/654814
Commit-Queue: Mike Frysinger <vapier@google.com>
Reviewed-by: Brian Gan <brgan@google.com>
Tested-by: Mike Frysinger <vapier@google.com>

manifest_xml.py

@@ -24,7 +24,7 @@ import xml.dom.minidom
 import error
 import git_config
 import git_refs
-from git_superproject import Superproject
+import git_superproject
 import platform_utils
 from project import Annotation
 from project import ManifestProject
@@ -1616,7 +1616,7 @@ https://gerrit.googlesource.com/git-repo/+/HEAD/docs/manifest-format.md
                         "no revision for superproject %s within %s"
                         % (name, self.manifestFile)
                     )
-                self._superproject = Superproject(
+                self._superproject = git_superproject.Superproject(
                     self,
                     name=name,
                     remote=remote.ToRemoteSpec(name),