fix(python): warn instead of failing on local imports in @udf source (#4318)

Since 0.40.0b8 (#4254), `@udf` raises when the packaged source imports a
module from a local source tree that is neither shipped with `code=` nor
named by a declared pip/conda package. That turned registrations that
used to succeed into errors:

- Callers that rewrite the packaged source before registering it now
fail at decoration. Sophon's datagen integration suite inlines its
test-module helpers this way, and
`test_registered_udf_source_stands_alone_on_a_worker` fails on 0.40.0b8.
- The check cannot know which modules a distribution installs. A
declared package whose distribution name differs from its module (for
example an internal library installed editable) is reported as local,
and there was no way to proceed.

This keeps the diagnosis but makes it a `UserWarning`, so registration
behaves as it did before #4254 and still tells the author, at
registration time, why the worker is likely to fail.

python/python/tests/test_function_packaging.py

@@ -11,6 +11,7 @@ import json
 import subprocess
 import sys
 import textwrap
+import warnings
 from pathlib import Path
 from typing import Annotated, Optional
 
@@ -299,9 +300,11 @@ def test_code_rejects_submodules_duplicates_and_non_modules(helper_package):
                 return value
 
 
-def test_local_module_outside_code_is_rejected_at_registration(
-    tmp_path, helper_package
-):
+def test_local_module_outside_code_warns_at_registration(tmp_path, helper_package):
+    """The worker most likely cannot import a local module that is neither
+    shipped nor declared, but registration still succeeds: a declared
+    distribution may install it under another name, and callers may rewrite
+    the packaged source before registering it."""
     helper_package(
         "fx_local", {"__init__.py": "def normalize(value):\n    return value\n"}
     )
@@ -322,11 +325,14 @@ def test_local_module_outside_code_is_rejected_at_registration(
         )
         return getattr(importlib.import_module(name), name)
 
-    with pytest.raises(ValueError, match=r"local module.*code=\[fx_local\]"):
-        define("fx_uses_local", "")
+    with pytest.warns(UserWarning, match=r"local module.*code=\[fx_local\]"):
+        local = define("fx_uses_local", "")
+    assert "from fx_local import normalize" in artifact(local).decode()
 
     # A declared package that provides the module is trusted to install it.
-    installed = define("fx_uses_installed", "pip=['fx-local==1.0']")
+    with warnings.catch_warnings():
+        warnings.simplefilter("error")
+        installed = define("fx_uses_installed", "pip=['fx-local==1.0']")
     assert "from fx_local import normalize" in artifact(installed).decode()