Allow for autoloading modules

Autoloads files within modules/ and makes them available by their module
type and name, e.g. `mixins-zsh`, `editor-nvim` and `users-opdavies`
when imported with `inputs.self.nixosModules`.

Based on afab322e6d/modules/default.nix.

This assumes there will be a modules/*.nix file and currently doesn't
work with modules/*/default.nix. This is something I'd like to add in
the future.

I also want find a cleaner way to pass arguments into these shortcuts as
this also doesn't work in their implementation as far as I can see.

modules/profiles/dwm.nix

@@ -0,0 +1 @@
+{ imports = [ ./dwm ]; }