[youtube] Extract formats from multiple DASH manifests (Closes #6093)

[youtube] Extract formats from multiple DASH manifests (Closes #6093)

DASH manifest pointed by dashmpd from the video webpage and one pointed by get_video_info may
be different (namely different itag set) - some itags are missing from DASH manifest pointed by
webpage's dashmpd, some - from DASH manifest pointed by get_video_info's dashmpd).
The general idea is to take a union of itags of both DASH manifests (for example video with such
'manifest behavior' see https://github.com/rg3/youtube-dl/issues/6093).