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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, calledarduino_deps.cpp (orarduino_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the library will fail to compile (differently from thedependencies field in thelibrary.properties that may become outdated if the author is not precise).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 anarduino_deps.cpp.
• Writing a correctarduino_deps.cpp may be tricky.
• An incorrectarduino_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 istitled accordingly?

Other information

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