[split_artists] Also fix alternate artist credits, not just primary

Migration SQL:

-- SET search_path=mbbot,musicbrainz;
DROP INDEX split_artists_history_artist_bot_version_time_idx;
ALTER TABLE split_artists_history ADD COLUMN credit int;
-- Previously we only looked at credits where artist.name=credit.name
-- Can't figure out credits for all rows, but we don't care about artists/credits that have disappeared
UPDATE split_artists_history sah SET credit=ac.id
    FROM artist_credit ac
    JOIN artist_credit_name acn ON (ac.id=acn.artist_credit)
    JOIN artist a ON (acn.artist=a.id)
    WHERE sah.credit IS NULL
      AND sah.artist=a.id
      AND ac.name=a.name;

VACUUM FULL split_artists_history;
CREATE INDEX split_artists_history_credit_bot_version_time_idx
    ON split_artists_history(credit, bot_version, time);