Importer: interactive option to show other releases from the same release group

I have an album called `Opeth - Garden Of The Titans`. This comes from a blu-ray with 2 CDs. They are on musicbrainz as well:

https://musicbrainz.org/release/bf3f9954-c048-4353-ae57-6160ab56d4b5

The original tags from the mp3s have a discnumber tag:

```
album: Garden Of The Titans
art: False
artist: Opeth
channels: 2
disc: 1
format: MP3
genre: Metal
genres: Metal
samplerate: 44100
title: Sorceress
track: 1
```

When I try to import this album to `beets`, this comes from Musicbrainz:

```
Correcting tags from:
Opeth - Garden Of The Titans
To:
Opeth - Garden of the Titans: Opeth Live at Red Rocks Amphitheatre
URL:
https://musicbrainz.org/release/c29d1500-04c0-4b59-9744-38eb41905e10
(Similarity: 86.7%) (album, tracks) (Digital Media, 2018, XW, Nuclear Blast)
* Ghost Of Perdition -> Ghost of Perdition
* Demon Of The Fall -> Demon of the Fall
* In My Time Of Need -> In My Time of Need
* The Devil's Orchard (#1) -> The Devil’s Orchard (#6) (index)
* Cusp Of Eternity (#2) -> Cusp of Eternity (#7) (index)
* Heir Apparent (#3) -> Heir Apparent (#8) (index)
* Era (#4) -> Era (#9) (index)
* Deliverance (#5) -> Deliverance (#10) (index)
[A]pply, More candidates, Skip, Use as-is, as Tracks, Group albums,
Enter search, enter Id, aBort? M
Finding tags for album "Opeth - Garden Of The Titans".
Candidates:
1. Opeth - Garden of the Titans: Opeth Live at Red Rocks Amphitheatre (86.7%) (album, tracks) (Digital Media, 2018, XW, Nuclear Blast)
2. LFO Demon / FFF - Clash of the Titans (25.5%) (tracks, artist, unmatched tracks, ...) (12" Vinyl, 2003, DE, Sprengstoff Recordings)
3. LFO Demon / FFF - Clash of the Titans (25.5%) (tracks, artist, unmatched tracks, ...) (Vinyl, 2003, DE, Mindbender Records)
4. Javier Navarrete - Wrath of the Titans (24.9%) (tracks, missing tracks, artist, ...) (CD, 2012, US, WaterTower Music)
5. Medusa - Clash of the Titans (23.0%) (tracks, artist, missing tracks, ...) (CD, 2005)
```

It's getting the digital release instead of the Blu-ray + 2 CDs.

If you search for "Garden of the Titans" in [musicbrainz interface](https://musicbrainz.org/search?query=garden+of+the+titans&type=release&method=indexed), you get 3 releases:

```
Garden of the Titans: Opeth Live at Red Rocks Amphitheatre | Opeth | Digital Media | 10 | 2018-11-02 | XW | Nuclear Blast | none | 727361435660 | eng / Latn | Album + Live | Official

Garden of the Titans: Opeth Live at Red Rocks Amphitheatre | Opeth | Blu-ray + 2×CD | 10 + 5 + 5 | 2018-11-02 | XE | Moderbolaget Records, Nuclear Blast Entertainment | 27361 43560, NE 4356-0 | 727361435608 | eng / Latn | Album + Live | Official

Garden of the Titans: Opeth Live at Red Rocks Amphitheatre | Opeth | Blu-ray + DVD-Video + 2×CD | 10 + 10 + 5 + 5 | 2018-11-02 | XW | Moderbolaget Records |   |   | eng / Latn | Album + Live | Official
```

But when searching like I did in the beginning of this issue, `beets` only get one release:

```
1. Opeth - Garden of the Titans: Opeth Live at Red Rocks Amphitheatre (86.7%) (album, tracks) (Digital Media, 2018, XW, Nuclear Blast)
```

Do we have a difference in the website search and musicbrainz API? Is this a musicbrainz or beets issue? How it decided that the best option was the digital download instead of the 2xCD since I have the discnumber tag in my mp3s?

Can we add support in Musicbrainz for `(see all versions of this release, X available)` similar to what you do to `More candidates`? There's this option in the (wrong) selected digital release at https://musicbrainz.org/release/c29d1500-04c0-4b59-9744-38eb41905e10, and if I could select other release available in the beets dialog would solve my problem.

Reproduce the importer flow for the Opeth release and inspect the candidate-selection interaction described in the issue. Compare the MusicBrainz release and release-group results involved in the search. Done means the interactive importer can expose and select other releases from the same release group, including the Blu-ray plus 2×CD release.

Written by the indexing model from the issue text.