qt_add_qml_module default value for OUTPUT_DIRECTORY creates conflicts

If you have qt_add_qml_module() 2 or more times and OUTPUT_DIRECTORY is not specified then you get an error when running CMake.

e.g.
 

qt_add_qml_module(CommonAnimals
    URI CommonAnimals
    VERSION 1.0
    SOURCES
        Animal.h Animal.cpp
        Mammal.h Mammal.cpp
        AlsoUsesAnimal.h AlsoUsesAnimal.cpp
        Swimming.h Swimming.cpp
)

qt_add_qml_module(Bird
    URI Bird
    VERSION 1.0
    SOURCES
        Penguin.h Penguin.cpp
)

fails to configure with the error:
 

CMake Error at /Users/paul.masristone/Qt/6.2.3/macos/lib/cmake/Qt6Qml/Qt6QmlMacros.cmake:305 (message):
  Output directory for target "Bird" is already used by another QML module
  (target "CommonAnimals").  Output directory is:

    /Users/paul.masristone/Code/build-Silly-Qt_6_2_3_for_macOS-Debug

Looking through the code in Qt6QmlMacros.cmake, I see that if OUTPUT_DIRECTORY is not provided, the default is the build root.

My suggestion to solve this issue is for the default to be changed to build-root/backing-target.

i.e. equivalent to me explicitly putting OUTPUT_DIRECTORY CommonAnimals and OUTPUT_DIRECTORY Bird respectively in my example above.

This would avoid directory clashes. I have been able to configure and build using this but there may be other side-effects I haven't considered.