manifest_xml: prevent extend-project from inheriting local groups

When extending a project in a local manifest, the project inherits the
`local:` group. This causes the superproject override logic (which omits
projects with `local:` groups) to incorrectly exclude the project from
the override manifest. This leads to "extend-project element specifies
non-existent project" errors during sync reload.

Fix this by stripping `local:` groups from extended projects, ensuring
they remain visible to superproject overrides while still allowing other
inherited groups to persist.

Bug: 470374343
Change-Id: I1a057ebffebc11a19dc14dde7cc13b9f18cdd0a3
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/543222
Reviewed-by: Peter Kjellerstedt <peter.kjellerstedt@axis.com>
Reviewed-by: Mike Frysinger <vapier@google.com>
Tested-by: Gavin Mak <gavinmak@google.com>
Commit-Queue: Gavin Mak <gavinmak@google.com>

manifest_xml.py

@@ -1487,6 +1487,14 @@ https://gerrit.googlesource.com/git-repo/+/HEAD/docs/manifest-format.md
                         continue
                     if groups:
                         p.groups |= groups
+                    # Drop local groups so we don't mistakenly omit this
+                    # project from the superproject override manifest.
+                    p.groups = {
+                        g
+                        for g in p.groups
+                        if not g.startswith(LOCAL_MANIFEST_GROUP_PREFIX)
+                    }
+
                     if revision:
                         if base_revision:
                             if p.revisionExpr != base_revision: