Currently, if a local manifest wants to add groups to an existing
project, it must use remove-project and then re-add the project with
the new groups. This makes the local manifest more fragile, requiring
updates to the local manifest if the original manifest changes.
Add a new extend-project tag, which supports adding groups to an
existing project.
Change-Id: Ib4d1352efd722a65dd263d02644b9ea5ab6ed400