Fix serialization of domain packages. (#20755)

Not all of their symbols are exported.

Author: hertschuh

keras/src/saving/serialization_lib.py

@@ -783,7 +783,8 @@ def _retrieve_class_or_fn(
 
         # Otherwise, attempt to retrieve the class object given the `module`
         # and `class_name`. Import the module, find the class.
-        if module == "keras.src" or module.startswith("keras.src."):
+        package = module.split(".", maxsplit=1)[0]
+        if package in {"keras", "keras_hub", "keras_cv", "keras_nlp"}:
             try:
                 mod = importlib.import_module(module)
                 obj = vars(mod).get(name, None)