The data_source penalty was not being calculated correctly because
`_get_distance` was being called for **all** enabled metadata plugins
which eventually meant that matches were being penalised needlessly.
This commit refactors the distance calculation to:
- Remove the plugin-based track_distance() and album_distance() methods
that were applying penalties incorrectly
- Calculate data_source penalties directly in track_distance() and
distance() functions when sources don't match
- Use a centralized get_penalty() function to retrieve plugin-specific
penalty values via a registry with O(1) lookup
- Change default data_source_penalty from 0.0 to 0.5 to ensure
mismatches are penalized by default
- Add data_source to get_most_common_tags() to determine the likely
original source for comparison
This ensures that tracks and albums from different data sources are
properly penalized during matching, improving match quality and
preventing cross-source matches.
