We can only do this now that every profile has been migrated, otherwise
we would get errors about undeclared modules... It's not perfect, but
it's good enough.
This is only done for `type == "nixos"` for now, as I don't have any
Darwin configurations...
This should define modules that are identical, or very similar.
The driving force is to be able to use `my.profiles` on home-manager and NixOS
without repeating myself.
In the future I might migrate other modules, such as `nixos/system/nix`...
