[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/normal.py

@@ -2,8 +2,10 @@
 
 
 class NormalPluginIE(InfoExtractor):
-    pass
+    _VALID_URL = 'normalpluginie'
+    REPLACED = False
 
 
 class _IgnoreUnderscorePluginIE(InfoExtractor):
+    _VALID_URL = 'ignoreunderscorepluginie'
     pass