[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/reload_plugins/yt_dlp_plugins/extractor/normal.py

@@ -0,0 +1,10 @@
+from yt_dlp.extractor.common import InfoExtractor
+
+
+class NormalPluginIE(InfoExtractor):
+    _VALID_URL = 'normal'
+    REPLACED = True
+
+
+class _IgnoreUnderscorePluginIE(InfoExtractor):
+    pass

test/testdata/reload_plugins/yt_dlp_plugins/postprocessor/normal.py

@@ -0,0 +1,5 @@
+from yt_dlp.postprocessor.common import PostProcessor
+
+
+class NormalPluginPP(PostProcessor):
+    REPLACED = True