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/hetznix/configuration.nix

@@ -1,11 +1,12 @@
 {
-  modulesPath,
+  inputs,
   lib,
+  modulesPath,
   pkgs,
   ...
 }:
 {
-  imports = [
+  imports = with inputs.self.nixosModules; [
     (modulesPath + "/installer/scan/not-detected.nix")
     (modulesPath + "/profiles/qemu-guest.nix")
 
@@ -16,7 +17,7 @@
     ./security
     ./services
 
-    ../../users/opdavies
+    ../../modules/users/opdavies.nix
   ];
 
   nix = {