When youtube-dl is not available in PATH, use yt-dlp instead

When youtube-dl is not available in PATH, use yt-dlp instead

Debian is going to drop youtube-dl which is not active upstream, and yt-dlp
is the replacement. This will make it be used if youtube-dl gets removed.

If an old version of youtube-dl remains installed, git-annex will still use
it. That might not be desirable, but changing git-annex to use yt-dlp in
preference to youtube-dl when both are installed risks breaking when
the user has annex.youtube-dl-options set to something that is supported
by youtube-dl, but not by yt-dlp.

Sponsored-by: Boyd Stephen Smith Jr. on Patreon