Make sure to find stack python before system python

[tmadlener]

Putting find_package(Gaudi) at the top makes that find python first, which will find the python from the Key4hep stack. Otherwise another package might find the system python first, which is not necessarily suitable for building this.

BEGINRELEASENOTES

• Make sure to prefer the Key4hep built python over the system python when calling cmake

ENDRELEASENOTES

[jmcarcell]

This will not work if headers are installed for the system python. I think there should be a better way because this happens from time to time in different repos. One option could be to set environment variables in the stack pointing to the python installation.
A change in the CMake config like this also works, but then this also has to be fixed for every repository:

execute_process(
    COMMAND which python3
    OUTPUT_VARIABLE PYTHON_FROM_PATH
    OUTPUT_STRIP_TRAILING_WHITESPACE
)
set(Python_EXECUTABLE ${PYTHON_FROM_PATH})
find_package(Python REQUIRED)

[tmadlener]

Yeah, it's a bit annoying. What if we put it into the common cmake config without find_package(Python)? If we ensure that that is called early enough, I think all subsequent find_package calls should do the right thing, and we have it centrally managed.