Allow use of a dependency helper source file during library detection

[cmaglie]

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• [ ] Tests for the changes have been added (for bug fixes / features)
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What kind of change does this PR introduce?

This is a tentative implementation for a strategy to increase the speed of the library discovery phase. The idea behind this PR is to add a helper file in a library, called arduino_deps.cpp (or arduino_deps.c), whose only purpose is to #include the required dependencies. If the Arduino CLI finds the helper the library detection process is performed only on that file, skipping all the other source files (that may be a lot...).

Advantages:

• It will correctly handle conditional includes (inside ifdefs).
• It must be maintained with the source code, otherwise, the library will fail to compile (differently from the dependencies field in the library.properties that may become outdated if the author is not precise).
• If a library has a lot of source files, the speed-up may be dramatic.

Disadvantages:

• It requires action from lib authors to write an arduino_deps.cpp.
• Writing a correct arduino_deps.cpp may be tricky.
• An incorrect arduino_deps.cpp may result in difficult-to-diagnose "missing include X.h" errors (that could be a workaround by including X.h in the user sketch)

What is the current behavior?

What is the new behavior?

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

Other information

This may be combined with #2625 for even better performance gain