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/hardware/default.nix

@@ -0,0 +1,14 @@
+# Hardware-related modules
+{ ... }:
+
+{
+  imports = [
+    ./bluetooth
+    ./ergodox
+    ./firmware
+    ./mx-ergo
+    ./networking
+    ./sound
+    ./upower
+  ];
+}