embedded platforms such as STM32
FYI, there is another thread about that: Generator for eclipse plugin for stm32 microcontrollers
I saw that toolchains and compiler are resolved using Modules/CMake.*.cmake files
Yes, though much of that is for command-line generators. For Visual Studio generators those modules have dedicated code paths, such as this one. In that case the modules are detecting what VS selects as a compiler rather than finding a compiler from the PATH or something.
which will sooner or later require something like
Windows-AVR-C.cmake
Yes. Those modules typically contain information specific to their platform/compiler/language combination. In your case if the toolchain can only be used with the new generator you may not need that much specific information in the corresponding modules and can instead hard-code it in the generator itself. You’ll have to choose which as things progress.