manifest_xml: group for submanifest projects

Add all projects in a submanifest to the group
submanifest::<path_prefix> for ease in filtering.

Change-Id: Ia6f01f9445f4f8d20fda3402f4d5821c43ceaf7f
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/331319
Tested-by: LaMont Jones <lamontjones@google.com>
Reviewed-by: Mike Frysinger <vapier@google.com>