[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

README.md

@@ -337,10 +337,11 @@ If you fork the project on GitHub, you can run your fork's [build workflow](.git
     --plugin-dirs PATH              Path to an additional directory to search
                                     for plugins. This option can be used
                                     multiple times to add multiple directories.
-                                    Note that this currently only works for
-                                    extractor plugins; postprocessor plugins can
-                                    only be loaded from the default plugin
-                                    directories
+                                    Use "default" to search the default plugin
+                                    directories (default)
+    --no-plugin-dirs                Clear plugin directories to search,
+                                    including defaults and those provided by
+                                    previous --plugin-dirs
     --flat-playlist                 Do not extract a playlist's URL result
                                     entries; some entry metadata may be missing
                                     and downloading may be bypassed