refactor: use 'default.nix' for all modules

modules/x/default.nix

@@ -0,0 +1,15 @@
+{ ... }:
+
+{
+  services.xserver.windowManager.xmonad = {
+    enable = true;
+    enableContribAndExtras = true;
+    config = builtins.readFile ../dots/.xmonad/xmonad.hs;
+  };
+
+  services.xserver = {
+    enable = true;
+    xkb.layout = "us";
+    xkb.variant = "";
+  };
+}