Omit local_manifest groups from superproject override.

When we create superproject_override.xml, do not include projects that
are present from local_manifests/*.  Such projects are fully under the
control of the local_manifests/ file.

Bug: b/238934278
Test: manual, ./run_tests
Change-Id: I40382ceb82d9cf7b8dc7b5f2abed3f6d4d80017e
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/340877


Tested-by: Xin Li <delphij@google.com>
Reviewed-by: Xin Li <delphij@google.com>
Reviewed-by: Sam Saccone  <samccone@google.com>