Move local-manifest check to manifest_xml.py

This removes the need for git_superproject to include manifest_xml, and
puts the logic for local_manifest detection in one place.

Change-Id: I4d33ded0542ceea4606a1ea24304f678de20c59e
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/330499
Tested-by: LaMont Jones <lamontjones@google.com>
Reviewed-by: Raman Tenneti <rtenneti@google.com>