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.

modules/system/default.nix

@@ -1,15 +0,0 @@
-# System-related modules
-{ ... }:
-
-{
-  imports = [
-    ./boot
-    ./docker
-    ./documentation
-    ./language
-    ./nix
-    ./packages
-    ./podman
-    ./users
-  ];
-}