Refactor of metadata plugin and opt in all metadata plugins to new baseclass (#5787)

At the moment the `MetaDataSourcePlugin` has multiple responsibilities:

- fetch data via `_search` api
- defines contract for interaction within the beets autotag lookup

I propose splitting these responsibilities, as it would enable us to use
the `MetaDataSourcePlugin` baseclass with plugins that use external
packages to fetch data.
  
This follows from discussion in #5761 and
#5748 (comment).

Feedback is highly appreciated, as this is mainly architectural decision
and I would prefer if the new behavior is a shared consensus.

## To Do

- [x] Opt in plugins into the new `MetaDataSourcePlugin` 
     - [x] Spotify
    - [x] Musicbrainz
    - [x] Deezer
    - [x] Beatport
    - [x] Chroma
    - [x] Disccogs
- [x] Remove old MetaDataSourcePlugin and related functions
- [x] Documentation on the ontology of plugins
- [x] Changelog


This PR was initially #5764 and was accidentally closed as the target
branch was deleted. Wasn't able to recover the original PR.

Author: snejus

beets/autotag/distance.py

@@ -8,7 +8,7 @@
 from jellyfish import levenshtein_distance
 from unidecode import unidecode
 
-from beets import config, plugins
+from beets import config, metadata_plugins
 from beets.util import as_string, cached_classproperty, get_most_common_tags
 
 if TYPE_CHECKING:
@@ -409,7 +409,7 @@ def track_distance(
         dist.add_expr("medium", item.disc != track_info.medium)
 
     # Plugins.
-    dist.update(plugins.track_distance(item, track_info))
+    dist.update(metadata_plugins.track_distance(item, track_info))
 
     return dist
 
@@ -526,6 +526,6 @@ def distance(
         dist.add("unmatched_tracks", 1.0)
 
     # Plugins.
-    dist.update(plugins.album_distance(items, album_info, mapping))
+    dist.update(metadata_plugins.album_distance(items, album_info, mapping))
 
     return dist

beets/autotag/match.py

@@ -24,7 +24,7 @@
 import lap
 import numpy as np
 
-from beets import config, logging, plugins
+from beets import config, logging, metadata_plugins
 from beets.autotag import AlbumInfo, AlbumMatch, TrackInfo, TrackMatch, hooks
 from beets.util import get_most_common_tags
 
@@ -119,7 +119,7 @@ def match_by_id(items: Iterable[Item]) -> AlbumInfo | None:
             return None
     # If all album IDs are equal, look up the album.
     log.debug("Searching for discovered album ID: {0}", first)
-    return plugins.album_for_id(first)
+    return metadata_plugins.album_for_id(first)
 
 
 def _recommendation(
@@ -274,7 +274,7 @@ def tag_album(
     if search_ids:
         for search_id in search_ids:
             log.debug("Searching for album ID: {0}", search_id)
-            if info := plugins.album_for_id(search_id):
+            if info := metadata_plugins.album_for_id(search_id):
                 _add_candidate(items, candidates, info)
 
     # Use existing metadata or text search.
@@ -311,7 +311,7 @@ def tag_album(
         log.debug("Album might be VA: {0}", va_likely)
 
         # Get the results from the data sources.
-        for matched_candidate in plugins.candidates(
+        for matched_candidate in metadata_plugins.candidates(
             items, search_artist, search_album, va_likely
         ):
             _add_candidate(items, candidates, matched_candidate)
@@ -346,7 +346,7 @@ def tag_item(
     if trackids:
         for trackid in trackids:
             log.debug("Searching for track ID: {0}", trackid)
-            if info := plugins.track_for_id(trackid):
+            if info := metadata_plugins.track_for_id(trackid):
                 dist = track_distance(item, info, incl_artist=True)
                 candidates[info.track_id] = hooks.TrackMatch(dist, info)
                 # If this is a good match, then don't keep searching.
@@ -372,7 +372,7 @@ def tag_item(
     log.debug("Item search terms: {0} - {1}", search_artist, search_title)
 
     # Get and evaluate candidate metadata.
-    for track_info in plugins.item_candidates(
+    for track_info in metadata_plugins.item_candidates(
         item, search_artist, search_title
     ):
         dist = track_distance(item, track_info, incl_artist=True)