Add `to-official` command

[nsensfel]

Add a to-official command that transforms all submodules into Git Submodules. This is likely to be challenging, since if that was simple, the git-submodules tool wouldn't have much of a reason to be there.
Beware of git-submodules that were previously Git Submodules and weren't turned into regular repositories beforehand.
At first glance, the steps to take are going to be:

1. Retrieve path list of all Git Submodules in the repository.
2. Remove from the repository all Git Submodules whose path was targeted by the to-official command.
3. Find a source that supports the target commit (Is that doable without cloning the repo?) for each target path.
4. Add a Git Submodule to the repository for each target path using the aforementioned source.