manifest_xml: allow src=. with symlinks

Some Android/Nest manifests are using <linkfile> with src="." to
create stable paths to specific projects.  Allow that specific
use case as it seems reasonable to support.

Bug: https://crbug.com/gerrit/11218
Change-Id: I5eadec257cd58ba0f8687c590ddc250a7a414a85
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/254276


Reviewed-by: Michael Mortensen <mmortensen@google.com>
Reviewed-by: Mike Frysinger <vapier@google.com>
Tested-by: Mike Frysinger <vapier@google.com>