Header Separation, main branch (2024.08.12.)

Separated the Acts, Gaudi, Geant4 and VecMem headers into separate subdirectories, while also updating FindGeoModel.cmake to work with the headers installed in such a way.

Also introduced a Findvecmem.cmake module to make sure that the runtime environment of vecmem would be set up correctly.

This is to make it possible to use an updated version of the above mentioned externals on top of an Athena nightly, building selected packages against an updated version of that external. (Without the build system getting confused where to pick up headers from.)

Tagging @elmsheus, @tsulaia and @boudreau for the "GeoModel aspect" of all of it. 😉