Added duduping on mb_trackid to the duplicates plugin

[tommyschnabel]

Description

plugins/duplicates: dedupe_mb_trackid_on_import option added to deduplicate during imports based on already-imported mb_trackid.

To Do

• Documentation. (If you've added a new command-line flag, for example, find the appropriate page under docs/ to describe it.)
• Changelog. (Add an entry to docs/changelog.rst to the bottom of one of the lists near the top of the document.)
• Tests. (Very much encouraged but not strictly required.)

[codecov]

Codecov Report

❌ Patch coverage is 20.83333% with 19 lines in your changes missing coverage. Please review.
✅ Project coverage is 72.01%. Comparing base (3d3f5e3) to head (50152df).

Files with missing lines	Patch %	Lines
beetsplug/duplicates.py	20.83%	19 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6313      +/-   ##
==========================================
- Coverage   72.07%   72.01%   -0.07%     
==========================================
  Files         159      159              
  Lines       20633    20656      +23     
  Branches     3273     3279       +6     
==========================================
+ Hits        14871    14875       +4     
- Misses       5053     5072      +19     
  Partials      709      709              

Files with missing lines	Coverage Δ
beetsplug/tidal/__init__.py	85.32% <ø> (ø)
beetsplug/duplicates.py	19.68% <20.83%> (-0.32%)	⬇️

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[snejus]

I think this addresses a real user problem, but it lands in the wrong architectural layer.

The duplicates plugin is currently scoped as a command plugin (beet duplicates) for inspecting and acting on duplicates already in the library (docs/plugins/duplicates.rst, beetsplug/duplicates.py). This PR adds import-time policy via import_task_created in beetsplug/duplicates.py, which changes importer behavior before core duplicate resolution runs.

Duplicate policy already has a clear owner in importer core: import.duplicate_keys, import.duplicate_action, and _resolve_duplicates() (docs/reference/config.rst, beets/ importer/stages.py). Splitting duplicate policy between importer core and the duplicates plugin will make behavior harder to reason about, harder to test, and less predictable for users.

Could we move this into importer core instead?

• Add importer-level support for track-level duplicate checks during album imports (config-driven).
• Reuse the existing duplicate action flow (skip/keep/remove/merge/ask) so UX and semantics stay consistent.
• Keep duplicates focused on post-import library analysis/actions via beet duplicates.

[tommyschnabel]

@snejus thanks for the feedback! Not super familiar with the entire project structure, so it's not surprising that this changeset doesn't fit in the duplicates plugin 😅

I'll move it to the importer 😄