Allow plugin extractors to replace the built-in ones

This allows easier plugin chaining; e.g.
- https://gist.github.com/pukkandan/24f13ff1ed385c5a390c1d7bd130d8f7
- https://gist.github.com/pukkandan/fcf5ca1785c80f64e471f0ee14f990fb

yt_dlp/extractor/extractors.py

@@ -3,6 +3,9 @@ import os
 
 from ..utils import load_plugins
 
+# NB: Must be before other imports so that plugins can be correctly injected
+_PLUGIN_CLASSES = load_plugins('extractor', 'IE', {})
+
 _LAZY_LOADER = False
 if not os.environ.get('YTDLP_NO_LAZY_EXTRACTORS'):
     with contextlib.suppress(ImportError):
@@ -19,5 +22,5 @@ if not _LAZY_LOADER:
     ]
     _ALL_CLASSES.append(GenericIE)  # noqa: F405
 
-_PLUGIN_CLASSES = load_plugins('extractor', 'IE', globals())
-_ALL_CLASSES = list(_PLUGIN_CLASSES.values()) + _ALL_CLASSES
+globals().update(_PLUGIN_CLASSES)
+_ALL_CLASSES[:0] = _PLUGIN_CLASSES.values()