Add a way to override the remote using <extend-project>

This commit supports for the 'remote' attribute in
<extend-project>. This avoids the need to perform a <remove-project>
followed by a <project> in local manifests.

Change-Id: I9f9347913337ec9d159bc264d15ce97881ae5398
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/253092


Tested-by: Kyunam Jo <kyunam.jo@gmail.com>
Reviewed-by: Mike Frysinger <vapier@google.com>