[extractor] Import _ALL_CLASSES lazily

This significantly speeds up `import yt_dlp` in the absence of `lazy_extractors`

yt_dlp/extractor/__init__.py

@@ -1,32 +1,15 @@
-import contextlib
-import os
+from ..compat.compat_utils import passthrough_module
 
-from ..utils import load_plugins
-
-_LAZY_LOADER = False
-if not os.environ.get('YTDLP_NO_LAZY_EXTRACTORS'):
-    with contextlib.suppress(ImportError):
-        from .lazy_extractors import *  # noqa: F403
-        from .lazy_extractors import _ALL_CLASSES
-        _LAZY_LOADER = True
-
-if not _LAZY_LOADER:
-    from ._extractors import *  # noqa: F403
-    _ALL_CLASSES = [  # noqa: F811
-        klass
-        for name, klass in globals().items()
-        if name.endswith('IE') and name != 'GenericIE'
-    ]
-    _ALL_CLASSES.append(GenericIE)  # noqa: F405
-
-_PLUGIN_CLASSES = load_plugins('extractor', 'IE', globals())
-_ALL_CLASSES = list(_PLUGIN_CLASSES.values()) + _ALL_CLASSES
+passthrough_module(__name__, '.extractors')
+del passthrough_module
 
 
 def gen_extractor_classes():
     """ Return a list of supported extractors.
     The order does matter; the first extractor matched is the one handling the URL.
     """
+    from .extractors import _ALL_CLASSES
+
     return _ALL_CLASSES
 
 
@@ -39,10 +22,12 @@ def gen_extractors():
 
 def list_extractor_classes(age_limit=None):
     """Return a list of extractors that are suitable for the given age, sorted by extractor name"""
+    from .generic import GenericIE
+
     yield from sorted(filter(
-        lambda ie: ie.is_suitable(age_limit) and ie != GenericIE,  # noqa: F405
+        lambda ie: ie.is_suitable(age_limit) and ie != GenericIE,
         gen_extractor_classes()), key=lambda ie: ie.IE_NAME.lower())
-    yield GenericIE  # noqa: F405
+    yield GenericIE
 
 
 def list_extractors(age_limit=None):
@@ -52,4 +37,6 @@ def list_extractors(age_limit=None):
 
 def get_info_extractor(ie_name):
     """Returns the info extractor class with the given ie_name"""
-    return globals()[ie_name + 'IE']
+    from . import extractors
+
+    return getattr(extractors, f'{ie_name}IE')