CPack Cleanup, master branch (2023.06.02.)

Simplified the CPack configuration in AtlasCMake a bit.

The goal is to try to ensure that RPM and TGZ packages would be created correctly (i.e. with the same layout as before) with either setting of ATLAS_USE_CUSTOM_CPACK_INSTALL_SCRIPT. Since as @miholzbo found a few weeks ago, RPMs are not correctly set up currently with ATLAS_USE_CUSTOM_CPACK_INSTALL_SCRIPT=FALSE.

🤞 that this will work correctly in all circumstances. I did a fair amount of testing, but we'll see... 😟