Bye bye legacy!

[alessio-perugini]

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• The PR has no duplicates (please search among the Pull Requests
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• The PR follows
  our contributing guidelines
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)
• UPGRADING.md has been updated with a migration guide (for breaking changes)
• configuration.schema.json updated if new parameters are added.

What kind of change does this PR introduce?

Removing the legacy pkg

What is the current behavior?

What is the new behavior?

Does this PR introduce a breaking change, and is titled accordingly?

Other information

[codecov]

Codecov Report

Patch coverage: 73.92% and project coverage change: -0.27% ⚠️

Comparison is base (6d57ce6) 63.31% compared to head (4e1afd1) 63.04%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2317      +/-   ##
==========================================
- Coverage   63.31%   63.04%   -0.27%     
==========================================
  Files         202      200       -2     
  Lines       19204    19206       +2     
==========================================
- Hits        12159    12109      -50     
- Misses       6015     6051      +36     
- Partials     1030     1046      +16     

Flag	Coverage Δ
unit	63.04% <73.92%> (-0.27%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files Changed	Coverage Δ
arduino/builder/builder.go	71.42% <69.87%> (-8.36%)	⬇️
arduino/builder/export_cmake.go	7.98% <75.00%> (ø)
arduino/builder/build_options_manager.go	80.19% <100.00%> (+0.40%)	⬆️
arduino/builder/core.go	73.84% <100.00%> (ø)
arduino/builder/libraries.go	81.25% <100.00%> (ø)
arduino/builder/linker.go	76.71% <100.00%> (-0.63%)	⬇️
arduino/builder/preprocess_sketch.go	100.00% <100.00%> (ø)
arduino/builder/recipe.go	76.31% <100.00%> (ø)
arduino/builder/sizer.go	47.59% <100.00%> (ø)
arduino/builder/sketch.go	63.24% <100.00%> (-0.32%)	⬇️
... and 1 more

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