qt6_add_qml_module only works when used in same folder than the backing library

If you try to use the qt6_add_qml_module function to create a plugin using an already existing library as a backing library and this library is build in a different folder (different CMakeLists.txt) the configure step fails with the following error:

CMake Error at /home/gagi/work/build/qt6/qtbase/lib/cmake/Qt6Core/Qt6CoreMacros.cmake:2020 (add_library):
  Cannot find source file:

    .../mytestlib_qmltyperegistrations.cpp

  Tried extensions .c .C .c++ .cc .cpp .cxx .cu .m .M .mm .h .hh .h++ .hm
  .hpp .hxx .in .txx

When using the qt6_add_qml_module function in the same CMakeLists.txt than the backing library it works as intended.

This is especially a problem when porting bigger qmake projects, as those usually have their libraries and plugins split into different folders and reusing this folder structure is not possible.