.prl file generation is dependent on add_subdirectory order

This is the root cause for QTBUG-89412.

Suppose you have two Qt modules:

add_subdirectory(3dextras)
add_subdirectory(3dinput)

and 3dextras depends on 3dinput such that the path to 3dinput's library appears in QMAKE_PRL_LIBS in the generated .prl file of 3dextras.

This currently doesn't work, because

• qt_generate_prl_file is called via scope finalizer "at the end of the sub-project file"
• add_subdirectory immediately evaluates the project file
• add_subdirectory(3dextras) does not see the targets that are created for 3dinput.

The work-around is to simply switch the order of the add_subdirectory calls:

add_subdirectory(3dinput)
add_subdirectory(3dextras)

The real fix is probably to move the qt_generate_prl_file call to QtPostProcess.cmake.