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/default.nix

@@ -0,0 +1,28 @@
+# Common modules
+{ lib, ... }:
+
+{
+  imports = [
+    ./hardware
+    ./home
+    ./programs
+    ./secrets
+    ./services
+    ./system
+  ];
+
+  options.my = with lib; {
+    user = {
+      name = mkOption {
+        type = types.str;
+        default = "ambroisie";
+        example = "alice";
+        description = "my username";
+      };
+
+      home = {
+        enable = my.mkDisableOption "home-manager configuration";
+      };
+    };
+  };
+}