lastgenre: Use multi-valued artist field for album matches too

Recently multi-valued artist field was put to use in the artist-genre-fetching-stage in `lastgenre`.

A minor note is that since it only tries to use the albumartists for the `fetch_artist_genre`, there could be a potential album match with `fetch_album_genre` for the individual artists (e.g. `fetch_album_genre(obj.albumartists[0], obj.album)`). Which might have more applicable tags than just falling back to the artists.

But I'm not sure how common that is, and the artist tags are plenty for my use case.

_Originally posted by @Nedra1998 in https://github.com/beetbox/beets/issues/5981#issuecomment-3667743909_

Search the lastgenre implementation and read the existing artist-genre-fetching-stage, especially fetch_artist_genre and fetch_album_genre. Check how the multi-valued albumartists field is currently handled, then make album matching consider the individual artists where appropriate. Done means album genre lookup can use those artist-specific album matches rather than only falling back to artist tags.

Written by the indexing model from the issue text.