Fetchart uses incorrect field for spotify_album_id

[arsaboo]

Fetchart currently uses album.mb_albumid to get spotify_album_id. This assumes mb_albumid has the spotify_album_id, which may be the case after the initial import with Spotify. However, mb_albumid can be overwritten with a valid mb_albumid, which will result in an error.

We should be using spotify_album_id instead, which will always have the right value.

[sampsyo]

Hi! Let's fold this into #604 and discuss it there. The one thing that makes this tricky is that deduplication is hard-coded to use the MBID field:

beets/beets/autotag/match.py

Lines 408 to 410 in 430a75d

	if info.album_id and info.album_id in results:
	log.debug('Duplicate.')
	return

Therefore, if there are multiple matches that are missing an ID, they will be considered duplicates of each other. We clearly need a way to identify duplicate matches of the same album (or track) in a source-scoped manner.