discogs: incorrectly interprets AA1, AA2 side labels as being a seperate disc.

I'm working on importing some of my vinyl rips to beets. The discogs plugin seems to be interpreting releases that have sides labeled as "AA1" as being double 12"s.

```sh
beet import '16bit - In The Death Car EP (2009) [Vinyl FLAC16]'/

/home/panamax/data/uploads/16bit - In The Death Car EP (2009) [Vinyl FLAC16] (3 items)

Match (100.0%):
16 Bit - In The Death Car EP
Discogs, 2xVinyl, 2009, UK, Boka Records, BOKA018, None
https://www.discogs.com/release/1610859-16-Bit-2-In-The-Death-Car-EP
≠ Artist: 16bit -> 16 Bit
* Album: In The Death Car EP
* Vinyl 1
* Vinyl 2
```
beets seems to be interpreting [this release](https://www.discogs.com/release/1610859-16-Bit-In-The-Death-Car-EP) as having 2 discs, when the release itself is only one disc.

A test with another two track AA side release yielded the same results.

```sh
beet import 'Edenhouse - Never Mind Modul #1 & #2 (1993) [Vinyl FLAC16]'/ -t

/home/panamax/data/torrents/uploads/Edenhouse - Never Mind Modul #1 & #2 (1993) [Vinyl FLAC16] (3 items)

Match (100.0%):
Edenhouse - Never Mind / Modul #1 & #2
Discogs, 2xVinyl, 1993, UK, Rising High Records, RSN 68, None
https://www.discogs.com/release/104216-Edenhouse-Never-Mind-Modul-1-2
* Artist: Edenhouse
* Album: Never Mind / Modul #1 & #2
* Vinyl 1: Other Side
* Vinyl 2: This Side
➜ [A]pply, More candidates, Skip, Use as-is, as Tracks, Group albums,
Enter search, enter Id, aBort?

```

Two tracks on a AA side isn't too common, but I was able to test with [another release]( https://www.discogs.com/release/78671-Weirdo-Normalisation) that has a single track on the AA side, without a number, and it imported that just fine. It doesn't present the issue with A1, A2, B

```sh
beet import 'Weirdo - Normalisation (Disc Two) (2002) [Vinyl FLAC16]'/ -t

/home/panamax/data/uploads/Weirdo - Normalisation (Disc Two) (2002) [Vinyl FLAC16] (2 items)

Match (97.6%):
Weirdo - Normalisation
≠ album
Discogs, Vinyl, 2002, UK, Tinrib Recordings, RIB 056, None
https://www.discogs.com/release/78671-Weirdo-Normalisation
* Artist: Weirdo
≠ Album: Normalisation (Disc Two) -> Normalisation
➜ [A]pply, More candidates, Skip, Use as-is, as Tracks, Group albums,
Enter search, enter Id, aBort?
```

Tracks with [A, AA, AB ](https://www.discogs.com/release/40491-Friends-Lovers-Family-Bob-Km) also do not present issues.

### Proposed Solution

I think this might be a case of needing to adjust the discogs.py:TRACK_INDEX_RE for track sides, which I'm happy to try my hand at fixing & testing.

Additionally, do a sanity check on import to make sure that the discs listed in the media field on discogs matches with the discs determined by the plugin before writing. If discogs.py thinks it has one discs, but discogs lists it as "Vinyl x2", or vice versa, might be a need for a source of truth as to disc count. Sometimes a multi-disc discogs release isn't properly listed with the number of discs, but I find that to be very rare on the site.

### Setup

* OS: Ubuntu 22.04.5 LTS
* Python version: 3.10.12
* beets version: 2.4.0
* Turning off plugins made problem go away (yes/no): no