Improve plugin architecture (#5553)

to make plugins easier to develop and use:
* Plugins are now loaded as namespace packages.
* Plugins can be loaded in any distribution of yt-dlp (binary, pip, source, etc.).
* Plugin packages can be installed and managed via pip, or dropped into any of the documented locations.
* Users do not need to edit any code files to install plugins.
* Backwards-compatible with previous plugin architecture.

As a side-effect, yt-dlp will now search in a few more locations for config files.

Closes https://github.com/yt-dlp/yt-dlp/issues/1389

Authored by: flashdagger, coletdjnz, pukkandan, Grub4K
Co-authored-by: Marcel <flashdagger@googlemail.com>
Co-authored-by: pukkandan <pukkandan.ytdlp@gmail.com>
Co-authored-by: Simon Sawicki <accounts@grub4k.xyz>

devscripts/make_lazy_extractors.py

@@ -40,8 +40,12 @@ def main():
 
     _ALL_CLASSES = get_all_ies()  # Must be before import
 
+    import yt_dlp.plugins
     from yt_dlp.extractor.common import InfoExtractor, SearchInfoExtractor
 
+    # Filter out plugins
+    _ALL_CLASSES = [cls for cls in _ALL_CLASSES if not cls.__module__.startswith(f'{yt_dlp.plugins.PACKAGE_NAME}.')]
+
     DummyInfoExtractor = type('InfoExtractor', (InfoExtractor,), {'IE_NAME': NO_ATTR})
     module_src = '\n'.join((
         MODULE_TEMPLATE,