Improve plugin architecture (#5553)

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 <redacted>
Co-authored-by: pukkandan <redacted>
Co-authored-by: Simon Sawicki <redacted>