Try harder to not recompile sketch without modifications.

[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?

Improves the checks made to the sketch to avoid useless recompilation and preprocessing.

What is the current behavior?

The sketch source code, before being compiled, undergoes a two-phase processing:

1. If the sketch has multiple source files, these files are merged, forming a single .cpp file.
2. The merged .cpp file is then "preprocessed" to automatically obtain the forward declarations of the functions defined in the sketch.

Previously, these two operations were always performed, touching the final sketch.cpp. This resulted in sketch.cpp being recompiled every time. Moreover, the library discovery cache could not be exploited because of the sketch.cpp timestamp change.

What is the new behavior?

The merged sketch file is saved in a separate sketch.cpp.merged file, and it is compared to the newly generated content. If they match, the already preprocessed sketch.cpp and sketch.cpp.merged are left untouched.
This is enough for the current build system to recognize that the file is already up-to-date and skip both compilation and library discovery, which greatly improves compile times.

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

Unless I'm missing some convoluted case, it shouldn't.

Other information

[codecov]

Codecov Report

❌ Patch coverage is 75.67568% with 54 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (master@3eecf20). Learn more about missing BASE report.
⚠️ Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
...l/arduino/builder/internal/detector/source_file.go	54.54%	19 Missing and 6 partials ⚠️
internal/arduino/builder/internal/utils/utils.go	56.52%	7 Missing and 3 partials ⚠️
...nternal/arduino/builder/internal/detector/cache.go	40.00%	4 Missing and 2 partials ⚠️
...rnal/arduino/builder/internal/detector/detector.go	81.48%	3 Missing and 2 partials ⚠️
...nal/arduino/builder/internal/preprocessor/ctags.go	71.42%	1 Missing and 3 partials ⚠️
internal/arduino/builder/sketch.go	66.66%	2 Missing and 1 partial ⚠️
...lder/internal/preprocessor/arduino_preprocessor.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##             master    #2961   +/-   ##
=========================================
  Coverage          ?   68.31%           
=========================================
  Files             ?      242           
  Lines             ?    22831           
  Branches          ?        0           
=========================================
  Hits              ?    15596           
  Misses            ?     6029           
  Partials          ?     1206           

Flag	Coverage Δ
unit	68.31% <75.67%> (?)

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