refactor: Deprecate extension submodule

Author: pawamoy

src/griffe_warnings_deprecated/extension.py

@@ -0,0 +1,18 @@
+"""Deprecated. Import from `griffe_warnings_deprecated` directly."""
+
+# YORE: Bump 2: Remove file.
+
+import warnings
+from typing import Any
+
+from griffe_warnings_deprecated._internal import extension
+
+
+def __getattr__(name: str) -> Any:
+    warnings.warn(
+        "Importing from `griffe_warnings_deprecated.extension` is deprecated. "
+        "Import from `griffe_warnings_deprecated` directly.",
+        DeprecationWarning,
+        stacklevel=2,
+    )
+    return getattr(extension, name)