[cleanup, extractor] Rename extractors.py to _extractors.py

This should be considered part of the next commit,
but is separated so that `git` can detect the renaming better

yt_dlp/extractor/__init__.py

@@ -11,7 +11,7 @@
         _LAZY_LOADER = True
 
 if not _LAZY_LOADER:
-    from .extractors import *  # noqa: F403
+    from ._extractors import *  # noqa: F403
     _ALL_CLASSES = [  # noqa: F811
         klass
         for name, klass in globals().items()