Only use primary artist aliases. Fixes #632.

MusicBrainz will send down various artist aliases that may not be the primary
(e.g. the previous name for a group). Only use an alias for the artist if it is
the primary alias in one of the import locales.
Update tests given new alias behavior
Merge pull request #633 from sampsyo/issue-632

Only use primary artist aliases. Fixes #632.