[desktop] Refactor into multiple components

roles/desktop/sddm.nix

@@ -0,0 +1,15 @@
+{ lib, config, pkgs, ... }:
+
+with lib;
+
+let
+  cfg = config.roles.desktop;
+in
+{
+  config = mkIf (cfg.enable && cfg.sddm) {
+    services.displayManager.sddm = {
+      enable = true;
+      wayland.enable = (!cfg.x11 && cfg.wayland);
+    };
+  };
+}