Fix Hybrid SACD importing (#5149)

Fixes #5148. 

When importing, the code that matches tracks does not consider the
medium number. This causes problems on Hybrid SACDs (and other releases)
where the artists, track numbers, titles, and lengths are the same on
both layers.

I added a distance penalty for mismatching medium numbers.

Before:

```
$ beet imp .

/Volumes/Music/ti/Red Garland/1958 - All Mornin' Long - 1 (6 items)

  Match (95.4%):
  The Red Garland Quintet - All Mornin' Long
  ≠ media, year
  MusicBrainz, 2xHybrid SACD (CD layer), 2013, US, Analogue Productions, CPRJ 7130 SA, mono
  https://musicbrainz.org/release/6a584522-58ea-470b-81fb-e60e5cd7b21e
  * Artist: The Red Garland Quintet
  * Album: All Mornin' Long
  * Hybrid SACD (CD layer) 1
     ≠ (#2-1) All Mornin' Long (20:21) -> (#1-1) All Mornin' Long (20:21)
     ≠ (#2-2) They Can't Take That Away From Me (10:24) -> (#1-2) They Can't Take That Away From Me (10:27)
     ≠ (#2-3) Our Delight (6:23) -> (#1-3) Our Delight (6:23)
  * Hybrid SACD (CD layer) 2
     ≠ (#1-1) All mornin' long (20:21) -> (#2-1) All Mornin' Long (20:21)
     ≠ (#1-2) They can't take that away from me (10:27) -> (#2-2) They Can't Take That Away From Me (10:25)
     ≠ (#1-3) Our delight (6:23) -> (#2-3) Our Delight (6:23)
➜ [A]pply, More candidates, Skip, Use as-is, as Tracks, Group albums,
Enter search, enter Id, aBort, eDit, edit Candidates?
```

Note that all tracks tagged with disc 1 get moved to disc 2 and vice
versa.

After:

```
$ beet-test imp .

/Volumes/Music/ti/Red Garland/1958 - All Mornin' Long - 1 (6 items)

  Match (95.4%):
  The Red Garland Quintet - All Mornin' Long
  ≠ media, year
  MusicBrainz, 2xMedia, 2013, US, Analogue Productions, CPRJ 7130 SA, mono
  https://musicbrainz.org/release/6a584522-58ea-470b-81fb-e60e5cd7b21e
  * Artist: The Red Garland Quintet
  * Album: All Mornin' Long
  * Hybrid SACD (CD layer) 1
     ≠ (#1-1) All mornin' long (20:21) -> (#1-1) All Mornin' Long (20:21)
     ≠ (#1-2) They can't take that away from me (10:27) -> (#1-2) They Can't Take That Away From Me (10:27)
     ≠ (#1-3) Our delight (6:23) -> (#1-3) Our Delight (6:23)
  * Hybrid SACD (SACD layer) 2
     * (#2-1) All Mornin' Long (20:21)
     * (#2-2) They Can't Take That Away From Me (10:24)
     * (#2-3) Our Delight (6:23)
➜ [A]pply, More candidates, Skip, Use as-is, as Tracks, Group albums,
Enter search, enter Id, aBort, eDit, edit Candidates?
```

Yay!

beets/autotag/match.py

@@ -208,6 +208,10 @@ def track_distance(
     if item.mb_trackid:
         dist.add_expr("track_id", item.mb_trackid != track_info.track_id)
 
+    # Penalize mismatching disc numbers.
+    if track_info.medium and item.disc:
+        dist.add_expr("medium", item.disc != track_info.medium)
+
     # Plugins.
     dist.update(plugins.track_distance(item, track_info))
 

beets/config_default.yaml

@@ -207,6 +207,7 @@ match:
         track_index: 1.0
         track_length: 2.0
         track_id: 5.0
+        medium: 1.0
     preferred:
         countries: []
         media: []

docs/changelog.rst

@@ -17,6 +17,9 @@ Bug fixes:
 * Check if running python from the Microsoft Store and provide feedback to install
   from python.org.
   :bug:`5467`
+* Fix bug where matcher doesn't consider medium number when importing. This makes
+  it difficult to import hybrid SACDs and other releases with duplicate tracks.
+  :bug:`5148`
 
 For packagers: