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.

extras/flakeModules.nix

@@ -16,6 +16,7 @@ let
       _file = "${toString moduleLocation}#flakeModules.${k}";
       key = "${toString moduleLocation}#flakeModules.${k}";
       imports = [ v ];
+      _class = "flake";
     });
     description = ''
       flake-parts modules for use by other flakes.