[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

test/testdata/yt_dlp_plugins/extractor/ignore.py

@@ -6,6 +6,7 @@ class IgnoreNotInAllPluginIE(InfoExtractor):
 
 
 class InAllPluginIE(InfoExtractor):
+    _VALID_URL = 'inallpluginie'
     pass
 
 

test/testdata/yt_dlp_plugins/extractor/normal.py

@@ -2,8 +2,10 @@ from yt_dlp.extractor.common import InfoExtractor
 
 
 class NormalPluginIE(InfoExtractor):
-    pass
+    _VALID_URL = 'normalpluginie'
+    REPLACED = False
 
 
 class _IgnoreUnderscorePluginIE(InfoExtractor):
+    _VALID_URL = 'ignoreunderscorepluginie'
     pass

test/testdata/yt_dlp_plugins/extractor/override.py

@@ -0,0 +1,5 @@
+from yt_dlp.extractor.generic import GenericIE
+
+
+class OverrideGenericIE(GenericIE, plugin_name='override'):
+    TEST_FIELD = 'override'

test/testdata/yt_dlp_plugins/extractor/overridetwo.py

@@ -0,0 +1,5 @@
+from yt_dlp.extractor.generic import GenericIE
+
+
+class _UnderscoreOverrideGenericIE(GenericIE, plugin_name='underscore-override'):
+    SECONDARY_TEST_FIELD = 'underscore-override'