Packaging changes:

• Remove run dependency of mpv package on mpv-libs - apparently mpv itself doesn't link against libmpv (it's provided only for external applications that want to leverage mpv's engine - and executable and library share a lot of common code).
• Add run dependencies on youtube-dl for both mpv and mpv-libs, as they both call youtube-dl to get stream links from URLs.
• Backport patches for fixes of slow caching from CIFS shares reported in T8962.

Depends on D9492, resolves T8962.

1. Played back local video file.
2. Played back streaming file.

Tests were performed without mpv-libs installed to make sure that mpv player itself doesn't need anything from libmpv.