Make all submodules have absolute URLs
The relative URLs were a workaround to let most contributors fork from Github due to a weakness in the haskell.org server. This workaround is no longer needed. And relative submodule URLs are an impediment to forking which makes contributions harder than they should be. The URLs are chosen to clone from https, because this makes sure that anybody, even not a registered Gitlab user, can clone a fork recursively.