[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

yt_dlp/globals.py

@@ -0,0 +1,30 @@
+from collections import defaultdict
+
+# Please Note: Due to necessary changes and the complex nature involved in the plugin/globals system,
+# no backwards compatibility is guaranteed for the plugin system API.
+# However, we will still try our best.
+
+
+class Indirect:
+    def __init__(self, initial, /):
+        self.value = initial
+
+    def __repr__(self, /):
+        return f'{type(self).__name__}({self.value!r})'
+
+
+postprocessors = Indirect({})
+extractors = Indirect({})
+
+# Plugins
+all_plugins_loaded = Indirect(False)
+plugin_specs = Indirect({})
+plugin_dirs = Indirect(['default'])
+
+plugin_ies = Indirect({})
+plugin_pps = Indirect({})
+plugin_ies_overrides = Indirect(defaultdict(list))
+
+# Misc
+IN_CLI = Indirect(False)
+LAZY_EXTRACTORS = Indirect(False)  # `False`=force, `None`=disabled, `True`=enabled