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/nixos/programs/default.nix

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