While porting qtquick3d to CMake, first qtshadertools had to be ported.

WIP change here: https://codereview.qt-project.org/c/qt/qtshadertools/+/289680

The qtshadertools repo uses provides a few qt libraries that use 3rdparty sources, and which are handled by qmake via the qt_helper_lib.prf feature.

So far the decision was not to support 3rdparty sources inside the qt repos, and instead depend on system installed ones (or vcpkg / conan provided ones).

Since then there have been discussions that it might be simpler to support building the 3rdparty deps from within the qt sources.

Also apparently for qtshadertools using system sources is not an option.

Thus we should add support for handling qt_helper_lib libraries in both QtBuild.cmake and pro2cmake.