quagga module: Use a deep merge via imports instead of the shallow merge

The deep merge caused all the options to be unset when generating docs, unless quagga was enabled.

Using imports, instead, properly allows the documentation to be generated.

nixos/modules/services/networking/quagga.nix

@@ -95,12 +95,10 @@ in
 {
 
   ###### interface
-
-  options.services.quagga =
+  imports = [
     {
-
+      options.services.quagga = {
         zebra = (serviceOptions "zebra") // {
-
           enable = mkOption {
             type = types.bool;
             default = any isEnabled services;
@@ -111,10 +109,11 @@ in
               if any routing protocols are configured.
             '';
           };
-
         };
-
-    } // (genAttrs services serviceOptions);
+      };
+    }
+    { options.services.quagga = (genAttrs services serviceOptions); }
+  ];
 
   ###### implementation