nixos: create 'modules/nixos' folder

Let's consolidate all modules under one path, so that NixOS,
home-manager, and nix-darwin (if I ever end up using it down the line)
would go under the same folder.

flake/nixos.nix

@@ -13,7 +13,7 @@ let
       ];
     }
     # Include generic settings
-    "${self}/modules"
+    "${self}/modules/nixos"
     # Include bundles of settings
     "${self}/profiles"
   ];