(sorry for the delay, there days are very busy for me)
@Jantje, you already answered yourself: including the following two paths
./libraries/Servo2/src
./libraries/Servo2/arch/${arch}
should solve your problem, is that a real burden?
Using a file like:
#ifdef ARDUINO_ARCH_AVR
#include ../arch/avr/Servo.cpp
#elif ARDUINO_ARCH_SAM
#include ../arch/avr/Servo.cpp
#else
#error The hardware is not supported
#endif
doesn't scale very well, every platform you're going to support need a separate #ifdef section to be added, and you should replicate this structure for every .c / .cpp file contained in your library, this means a quadratic load of maintenance work (N_of_files * N_of_platforms) just to make it compile.
- It is not obvious from the code what is going on.
- There is no easy way to notice a hardware is not supported.
It's written into "platforms" field of library.properties, IMHO is much more clearer than digging into the code
The goal is to allow to compile the same sketch (without changes) across multiple architectures so, to allow this, there should be a common header and multiple implementations (one for each architecture).
Personally I don't like to duplicate a library into many different places, because this leads to have only a subset of them updated. I've experienced this with many Arduino libraries: some contributions slightly change the API (=> the common headers) on the AVR side but doesn't care to fix things for the less popular SAM.
BTW, if you like to use the libraries folder as you do with energia, I don't see any reason to not continue to do so.