FindGaudi.cmake Fixes, main branch (2024.09.04.)

In order to handle the many issues that showed up in athena!74063 (merged), ended up modernizing FindGaudi.cmake a bit.

Switched to setting up the GaudiKernel target as an alias, while making sure that the GaudiKernel headers would be treated as system headers. (Previously this behaviour came to most Athena packages unintentionally, by picking up other headers from AthenaExternals as well.)

Found that the Gaudi installation finding CLHEPConfig.cmake internally was causing some nasty effects on AtlasLCG's ability to use its own FindCLHEP.cmake code. Resulting in some really weird hysteresis behaviour in what CMake would do when configuring an ATLAS project. This is fixed by forefully removing the variables at the end of FindGaudi.cmake that would make FindCLHEP.cmake confused.