Add mbpseudo plugin for pseudo-release proposals (#5888)

## Description
Adds the new `mbpseudo` plugin, that proactively searches for pseudo-releases during import and
adds them as candidates. Since it also depends on MusicBrainz, there are
some special considerations for the default logic (which is now a plugin
as well). However, at the very least it expects a list of desired [names
of scripts](https://en.wikipedia.org/wiki/ISO_15924) in the
configuration, for example:

```yaml
mbpseudo:
    scripts:
    - Latn
```

It will use that to search for pseudo-releases that match some of the
desired scripts, but will only do so if the input tracks match against
an official release that is not in one of the desired scripts.

## Standalone Usage

This would be the recommended approach, which involves disabling the
`musicbrainz` plugin. The `mbpseudo` plugin will manually delegate the
initial search to it. Since the data source of official releases will
still match MusicBrainz, weights are still relevant:

```yaml
mbpseudo:
    source_weight: 0.0
    scripts:
    - Latn

musicbrainz:
    source_weight: 0.1
```

A setup like that would ensure that the pseudo-releases have slightly
more preference when choosing the final proposal.

## Combined Usage

I initially thought it would be important to coexist with the
`musicbrainz` plugin when it's enabled, and reuse as much of its data as
possible to avoid redundant calls to the MusicBrainz API. I have the
impression this is not really important in the end, and maybe things
could be simplified if we decide that both plugins shouldn't coexist.

As it is right now, using both plugins at the same time would still
work, but it'll only avoid redundancy if `musicbrainz` emits its
candidates before `mbpseudo`, ~which is why I modified the
plugin-loading logic slightly to guarantee ordering. I'm not sure if you
think this could be an issue, but I think the `musicbrainz` plugin is
also used by other plugins and I can imagine it's good to guarantee the
order that is declared in the configuration?~

If the above is fulfilled, the `mbpseudo` plugin will use listeners to
intercept data emitted by the `musicbrainz` plugin and check if any of
them have pseudo-releases that might be desirable.

Author: semohr

.github/CODEOWNERS

@@ -2,4 +2,5 @@
 * @beetbox/maintainers
 
 # Specific ownerships:
-/beets/metadata_plugins.py @semohr
+/beets/metadata_plugins.py @semohr
+/beetsplug/mbpseudo.py @asardaes

beets/autotag/match.py

@@ -24,7 +24,7 @@
 import lap
 import numpy as np
 
-from beets import config, logging, metadata_plugins
+from beets import config, logging, metadata_plugins, plugins
 from beets.autotag import AlbumInfo, AlbumMatch, TrackInfo, TrackMatch, hooks
 from beets.util import get_most_common_tags
 
@@ -274,12 +274,17 @@ def tag_album(
             log.debug("Searching for album ID: {}", search_id)
             if info := metadata_plugins.album_for_id(search_id):
                 _add_candidate(items, candidates, info)
+                if opt_candidate := candidates.get(info.album_id):
+                    plugins.send("album_matched", match=opt_candidate)
 
     # Use existing metadata or text search.
     else:
         # Try search based on current ID.
         if info := match_by_id(items):
             _add_candidate(items, candidates, info)
+            for candidate in candidates.values():
+                plugins.send("album_matched", match=candidate)
+
             rec = _recommendation(list(candidates.values()))
             log.debug("Album ID match recommendation is {}", rec)
             if candidates and not config["import"]["timid"]:
@@ -313,6 +318,8 @@ def tag_album(
             items, search_artist, search_album, va_likely
         ):
             _add_candidate(items, candidates, matched_candidate)
+            if opt_candidate := candidates.get(matched_candidate.album_id):
+                plugins.send("album_matched", match=opt_candidate)
 
     log.debug("Evaluating {} candidates.", len(candidates))
     # Sort and get the recommendation.

beets/plugins.py

@@ -72,6 +72,7 @@
     "album_imported",
     "album_removed",
     "albuminfo_received",
+    "album_matched",
     "before_choose_candidate",
     "before_item_moved",
     "cli_exit",