project: 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: I16dbe8d9fe42ea45440afcb61404c753bff1930d
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/254330


Reviewed-by: Chanho Park <parkch98@gmail.com>
Reviewed-by: Mike Frysinger <vapier@google.com>
Tested-by: Mike Frysinger <vapier@google.com>