legacy: Make a lot of Builder internals private

[cmaglie]

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• The PR has no duplicates (please search among the Pull Requests
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• The PR follows
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• 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?

It made most of the builder internals private. It will allow future refactorings without worrying about breaking changes in the golang API.

What is the current behavior?

What is the new behavior?

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

Other information

[alessio-perugini]

Some tests are failing

[codecov]

Codecov Report

Patch coverage: 76.47% and no project coverage change.

Comparison is base (0e6133f) 63.03% compared to head (873d1d3) 63.04%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2325   +/-   ##
=======================================
  Coverage   63.03%   63.04%           
=======================================
  Files         201      201           
  Lines       19264    19260    -4     
=======================================
- Hits        12143    12142    -1     
+ Misses       6067     6065    -2     
+ Partials     1054     1053    -1     

Flag	Coverage Δ
unit	63.04% <76.47%> (+<0.01%)	⬆️

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

Files Changed	Coverage Δ
arduino/builder/core.go	73.84% <ø> (ø)
arduino/builder/export_cmake.go	7.98% <ø> (ø)
arduino/builder/internal/compilation/database.go	66.66% <ø> (ø)
arduino/builder/internal/detector/detector.go	74.34% <ø> (ø)
arduino/builder/internal/logger/logger.go	85.00% <ø> (ø)
...lder/internal/preprocessor/arduino_preprocessor.go	0.00% <ø> (ø)
arduino/builder/internal/preprocessor/ctags.go	68.62% <ø> (ø)
arduino/builder/internal/preprocessor/gcc.go	79.54% <ø> (ø)
...al/preprocessor/internal/ctags/ctags_has_issues.go	98.58% <ø> (ø)
...ternal/preprocessor/internal/ctags/ctags_parser.go	98.01% <ø> (ø)
... and 12 more

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