avformat/hls: decouple playlists from variants

Not all "sub-playlists" are variant playlists (containing the same
content with a different bitrate, etc) in the current version of the HLS
specification. They can now also be alternative renditions, containing
e.g. alternative audio tracks etc.

Decouple playlists from variants to prepare for handling the new
features.
Signed-off-by: Anssi Hannula <anssi.hannula@iki.fi>