submodules: Use a shorter relative path to gvdb

This improves usage of custom Git URL shorthands; for example allowing cloning using the gnome:glib URL and a configuration like this in the user's .gitconfig:

[url "https://gitlab.gnome.org/GNOME/"] insteadof = gnome:

Without this patch, you'd get this ever so cryptic SSH error:

ssh: Could not resolve hostname .: Name or service not known

This also slightly lowers the dependency on projects layout as well, as it only requires the repository to be at the same hierarchical level rather than in a GNOME/ sub-directory two levels up.