Draft: Test upcoming CMake change in ROOT.

Test https://github.com/root-project/root/pull/8709

This PR converts several dependencies of ROOT from variables with strings to CMake targets. This should mostly affect builtins, but better to test that dependencies are still found correctly.

To run the testing, add a comment with:

Bot test PACKAGE[ PACKAGE2 ...]|all in TOOLCHAIN|all [(only|also) for ARCH-LABEL-COMPILER-BUILDTYPE] please!

later followed by

Bot repeat please!

To ensure patches required for existing LCG branches are not lost, please add the respective labels: alsoTargeting:LCG_102-patches, alsoTargeting:master, etc.