Move Distance to a dedicated module and refactor related tests (#5800)

This PR:

1. Reorganizes distance-related code by moving it from `hooks.py` and
`match.py` to a new dedicated `distance.py` module:
- The actual implementation logic and algorithms remain unchanged - code
is moved, not rewritten
- Distance class, string distance functions, and track/album distance
calculators are relocated intact
- Only imports and function references are updated to maintain
compatibility
- `current_metadata` function is replaced with equivalent
`get_most_common_tags` function for clarity

2. Refactors the distance testing code from unittest to pytest:
- Tests now use fixtures and parametrization while verifying the same
functionality
- The tested behaviors remain identical, just with improved test
structure
- Actually, `distance.py` coverage slightly increased since I included
an additional test

3. Adds a test for the `sanitize_pairs` function to complete config
utility test coverage

This is primarily a code organization improvement that follows better
separation of concerns, grouping related distance functionality in a
single module without changing how the distance calculations work. No
algorithm changes or behavior modifications were made to the core
distance calculation code - it was simply moved to a more appropriate
location.

Author: snejus

beets/autotag/__init__.py

@@ -14,36 +14,37 @@
 
 """Facilities for automatically determining files' correct metadata."""
 
-from collections.abc import Mapping, Sequence
-from typing import Union
+from __future__ import annotations
+
+from typing import TYPE_CHECKING, Union
 
 from beets import config, logging
-from beets.library import Album, Item, LibModel
+from beets.util import get_most_common_tags as current_metadata
 
 # Parts of external interface.
 from beets.util import unique_list
 
-from .hooks import AlbumInfo, AlbumMatch, Distance, TrackInfo, TrackMatch
-from .match import (
-    Proposal,
-    Recommendation,
-    current_metadata,
-    tag_album,
-    tag_item,
-)
+from .distance import Distance
+from .hooks import AlbumInfo, AlbumMatch, TrackInfo, TrackMatch
+from .match import Proposal, Recommendation, tag_album, tag_item
+
+if TYPE_CHECKING:
+    from collections.abc import Mapping, Sequence
+
+    from beets.library import Album, Item, LibModel
 
 __all__ = [
     "AlbumInfo",
     "AlbumMatch",
-    "Distance",
-    "TrackInfo",
-    "TrackMatch",
+    "Distance",  # for backwards compatibility
     "Proposal",
     "Recommendation",
+    "TrackInfo",
+    "TrackMatch",
     "apply_album_metadata",
     "apply_item_metadata",
     "apply_metadata",
-    "current_metadata",
+    "current_metadata",  # for backwards compatibility
     "tag_album",
     "tag_item",
 ]