[zakharstudenttuke]

Fixes #7040.

Adds an optional format_map setting to the convert plugin. It maps the format of a source file to the format it should be converted to, so one run can produce different formats depending on the input:

convert:
  format: mp3
  format_map:
    alac: flac
    wav: flac

How the target format of a file is chosen:

1. -f/--format on the command line, if given (overrides everything).
2. format_map, using the format of the source file.
3. The existing format option.

Implementation notes:

• The target format and command were computed once per run (fmt, command). They are now looked up per item (get_target_format, get_command), and should_transcode, get_item_destination, convert_item, the playlist code and convert_on_import use them.
• The -f/--format option no longer has the config value as its default. It is None unless given, so the plugin can tell "not given" from "given". The option is otherwise unchanged.
• Keys of format_map are case-insensitive. wav is accepted as an alias of WAVE, which is how beets names WAV files.
• Without format_map the behaviour is unchanged.
• A source file that is already in its target format is copied, as before.

Testing:

• New tests in test/plugins/test_convert.py (the existing stub command is used, so ffmpeg is not needed).
• I also ran it with real ffmpeg on wav/flac/ogg/mp3 files and checked the codecs of the results with ffprobe.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.47%. Comparing base (929788f) to head (ee8a3e7).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7108      +/-   ##
==========================================
- Coverage   80.49%   80.47%   -0.03%     
==========================================
  Files         149      149              
  Lines       21248    21259      +11     
  Branches     3282     3283       +1     
==========================================
+ Hits        17104    17108       +4     
- Misses       3315     3321       +6     
- Partials      829      830       +1     

Files with missing lines	Coverage Δ
beetsplug/convert.py	76.75% <100.00%> (+0.71%)	⬆️

... and 2 files with indirect coverage changes

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