nixos/ddccontrol: add package option

nixos/modules/services/hardware/ddccontrol.nix

@@ -22,6 +22,13 @@ in
         This [enables `hardware.i2c`](#opt-hardware.i2c.enable), so note to add
         yourself to [`hardware.i2c.group`](#opt-hardware.i2c.group).
       '';
+      package =
+        lib.mkPackageOption pkgs
+          "package with which to control brightness; added also to [services.dbus.packages](#opt-services.dbus.packages)."
+          {
+            default = [ "ddccontrol" ];
+            example = [ "ddcutil-service" ];
+          };
     };
   };
 
@@ -36,17 +43,16 @@ in
       enable = true;
     };
 
-    # Give users access to the "gddccontrol" tool
     environment.systemPackages = [
-      pkgs.ddccontrol
+      cfg.package
     ];
 
     services.dbus.packages = [
-      pkgs.ddccontrol
+      cfg.package
     ];
 
     systemd.packages = [
-      pkgs.ddccontrol
+      cfg.package
     ];
   };
 }