Add class: imports "type checking"

Initially this had some feature checking with lib.functionArgs, but
I don't think this is useful. The _class attribute is not supported
by long-unsupported Nixpkgs (<23.05) anyway, so let's keep it simple.

modules/perSystem.nix

@@ -126,6 +126,7 @@ in
           specialArgs = {
             inherit system;
           };
+          class = "perSystem";
         }).config;
     };