Main CMake scripts cleanup
I've split the changes to files that are not the top level CMake script, kate's CMake script or kwrite's CMake script into separate commits.
These changes shouldn't change any functionality except the output produced by CMake's configure stage. I made every find_package call QUIET since we print a summary at the end of the configure stage anyway but there's still some find_package output appearing in the output which I suspect is produced because some dependencies use find_package instead of find_dependency in their CMake config files which causes QUIET to not get propagated.
The commit messages list all of the changes made. There are quite a few changes so it's probably a good idea to verify everything still works on a machine other than my own before merging.