Merge pull request #4587 from RosettaCommons/roccomoretti/submodule_relative
Convert .gitmodules to use relative paths.
Right now we're hard coding SSH paths for various submodules, which is annoying if people cloned main using https, especially now that we have submodules which are required for compilation.
It turns out, though, that Git can use relative paths for submodules, which allows the submodules to be cloned using the same method the main repo was.