Handle correctly QT_PRIVATE assignments when converting to CMake

As uncovered by the failing qtquickcontrols2 tst_focus, we don't correctly handle QT_PRIVATE assignments when converting to CMake.

Any module that has QT_PRIVATE += opengl-private assignment we currently interpret as

qt_extend_target(Quick LIBRARIES Qt::OpenGLPrivate)

which under the hood in a loop in qt_extend_target makes QuickPrivate depend on OpenGLPrivate, which is incorrect.

Instead we need to change pro2cmake to generate code like his for the following .pro snippet:

TARGET = quick
QT += gui-private
QT_PRIVATE += opengl-private

# Make Quick depend on QuickPrivate privately in add_qt_module(Quick) in QtBuild.cmake, and do the following

target_link_libraries(QuickPrivate
   INTERFACE Qt::GuiPrivate
)
target_link_libraries(Quick
   PRIVATE Qt::OpenGLPrivate
)

Note the usage of a simple target_link_libraries instead of qt_extend_target.