musicbrainz: Use artist alias closest to tags

### Problem
If an artist use many aliases, the importing and auto-tagging functionality of beets and musicbrianz suggest the main artist name only, trying to fix the existing alias. As the alias was officially used, it should be retained.
Suggestion: keep the artist alias closes to the one present in tags.

Led to this problem:
Example: the band KLF used many other names incl. 2K. [This release](https://www.discogs.com/2K-k-The-Millennium/release/127207) is identified but forced to change the artist, despite the fact that the referenced musicbrainz URL is linked with the alias.

```
Correcting tags from:
2k - ***k the millennium
To:
The KLF - ***k the Millennium
URL:
http://musicbrainz.org/release/3ec83d1e-4f91-48aa-89dd-ac3bd75dd35a
```

### Setup

* OS: debian
* Python version: 2.7.9
* beets version: 1.4.2
* Turning off plugins made problem go away (yes/no): no

My configuration (output of `beet config`) is:

```yaml
directory: /z/sounds_the_rest
library: /h/beets/library.db
asciify_paths: yes
format_album: %lower{$albumartist} - %lower{$album}
import:
copy: no
write: no
plugins: discogs
```

Start by reproducing the import with the provided MusicBrainz release URL and configuration, then trace where artist aliases are selected during matching and tag correction. Done means an officially used alias closest to the existing tag is retained instead of always replacing it with the main artist name.

Written by the indexing model from the issue text.