Header Installation Fix, 1.0 branch (2023.04.21.)

Made atlas_install_headers(...) more robust, making it work correctly with the CMake default installation method as well. (Not just with the ATLAS specific cpack_install.sh.in script.)

This was necessitated by !1009 (merged), which broke the installation of the header directory when -DATLAS_USE_CUSTOM_CPACK_INSTALL_SCRIPT=FALSE was used.

For @robouque's benefit: The way that I tested these changes with your robouque/PrunTarballCreation repository was that I checked out your repository and this repository side-by-side, and then set up the build of your code with:

cmake -DAtlasCMake_DIR=/srv/atlasexternals/Build/AtlasCMake \
   -DLCG_DIR=/srv/atlasexternals/Build/AtlasLCG \
   -DATLAS_USE_CUSTOM_CPACK_INSTALL_SCRIPT=<TRUE/FALSE> \
   ../PrunTarballCreation/source/

(This was in a Singularity container, hence the /srv path.)

Note that this replaces !1011 (closed).