[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/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'