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.

hosts/t490/configuration.nix

@@ -1,7 +1,7 @@
-{ pkgs, ... }:
+{ inputs, pkgs, ... }:
 
 {
-  imports = [
+  imports = with inputs.self.nixosModules; [
     ./hardware-configuration.nix
     ./hardware.nix
     ./programs.nix
@@ -9,9 +9,9 @@
     ./services
     ./users.nix
 
-    ../../modules/mixins/common
+    mixins-common
 
-    ../../users/opdavies
+    users-opdavies
   ];
 
   boot = {