Improve support for submodules

It seems like changing the source url for a submodule is conceptually broken. Part of it is how git handles submodules (via a file), and part of it is how we're doing forks. When I fork my project, I typically fork 4 repos, a main one and 3 submodules. When I change the submodule source url to the forked one it marks the .gitmodule file as modified; which is fine for my local repo but shouldn't be pushed in that way.

There should be an easier way, perhaps set it up as a remote?