Getting prepared for relative submodule specs

.gitmodules file may have relative submodule paths instead of full
url.

Change-Id: Ib4f00cb2a75a00ffdb7465b669354e0bf7bab1f9
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
Reviewed-by: Tony Sarajärvi <tony.sarajarvi@digia.com>
Reviewed-by: Janne Anttila <janne.anttila@digia.com>