[core] Load plugins on demand (#11305)

- Adds `--no-plugin-dirs` to disable plugin loading
- `--plugin-dirs` now supports post-processors

Authored by: coletdjnz, Grub4K, pukkandan

yt_dlp/extractor/__init__.py

@@ -1,16 +1,25 @@
 from ..compat.compat_utils import passthrough_module
+from ..globals import extractors as _extractors_context
+from ..globals import plugin_ies as _plugin_ies_context
+from ..plugins import PluginSpec, register_plugin_spec
 
 passthrough_module(__name__, '.extractors')
 del passthrough_module
 
+register_plugin_spec(PluginSpec(
+    module_name='extractor',
+    suffix='IE',
+    destination=_extractors_context,
+    plugin_destination=_plugin_ies_context,
+))
+
 
 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
+    import_extractors()
+    return list(_extractors_context.value.values())
 
 
 def gen_extractors():
@@ -37,6 +46,9 @@ def list_extractors(age_limit=None):
 
 def get_info_extractor(ie_name):
     """Returns the info extractor class with the given ie_name"""
-    from . import extractors
+    import_extractors()
+    return _extractors_context.value[f'{ie_name}IE']
 
-    return getattr(extractors, f'{ie_name}IE')
+
+def import_extractors():
+    from . import extractors  # noqa: F401