Rename json.py to _json.py

To make it explicit that it is a private implementation detail.

Author: sbidoul

src/pip/_internal/metadata/json.py renamed to src/pip/_internal/metadata/_json.py

@@ -37,7 +37,7 @@
 from pip._internal.utils.misc import is_local, normalize_path
 from pip._internal.utils.urls import url_to_path
 
-from .json import msg_to_json
+from ._json import msg_to_json
 
 if TYPE_CHECKING:
     from typing import Protocol