[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/postprocessor/normal.py

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