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/programs/default.nix

@@ -1,8 +0,0 @@
-# Program-related modules
-{ ... }:
-
-{
-  imports = [
-    ./steam
-  ];
-}