[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/utils/_utils.py

@@ -52,6 +52,7 @@ from ..compat import (
     compat_HTMLParseError,
 )
 from ..dependencies import xattr
+from ..globals import IN_CLI
 
 __name__ = __name__.rsplit('.', 1)[0]  # noqa: A001: Pretend to be the parent module
 
@@ -1487,8 +1488,7 @@ def write_string(s, out=None, encoding=None):
 
 # TODO: Use global logger
 def deprecation_warning(msg, *, printer=None, stacklevel=0, **kwargs):
-    from .. import _IN_CLI
-    if _IN_CLI:
+    if IN_CLI.value:
         if msg in deprecation_warning._cache:
             return
         deprecation_warning._cache.add(msg)
@@ -4891,10 +4891,6 @@ class Config:
     filename = None
     __initialized = False
 
-    # Internal only, do not use! Hack to enable --plugin-dirs
-    # TODO(coletdjnz): remove when plugin globals system is implemented
-    _plugin_dirs = None
-
     def __init__(self, parser, label=None):
         self.parser, self.label = parser, label
         self._loaded_paths, self.configs = set(), []