Export versionless config files and targets that can be used by applications

This will provide applications built using CMake and Qt5, to gradually / more easily update to Qt6.

This task involves a few things:
1) Provide packages that contain both versionless and major-version targets, so find_package(Qt5 Components Core) will provide Qt5::Core and Qt::Core.
2) Allow the application developer to opt in whether versionless targets should be created by the find_package call (via setting a specific variable before the find_package call)
3) Provide macros and functions in both variants as well (version less and with major version, e.g. qt_automoc() and qt5_automoc())

Point (1) is implemented. (2) and (3) still need to be done.