Support absolute URLs to submodules

Description

The addition of GIT_SUBMODULE_STRATEGY in !443 (merged) is a nice feature. However, I'm finding it awkward to meet the constraint that URLs in .gitmodules must be relative. I'm working on a project where having the same relative path to a submodule in development is not practical (parent project is in my GOPATH, submodule is not). Requiring that all developers set up their local clones with the same relative path as the projects on GitLab is not working.

My goals is to be able to use a git or https URL for a submodule that is a non-publicly accessible repository on the same GitLab server as the parent repo.

Proposal

Ideally, I could have git or https URLs in a .gitmodules, and the runner would figure out if there is a relative path between the parent project and the submodule.

If this is not practical, it would be nice to have a hook where I could run a script to modify the .gitmodules URLs before the submodule init/update.