Restrict qt_target_qml_sources usage to real QML modules only

Context
https://git.qt.io/alcroito/qt6-cmake-api-review/-/commit/14b4999bc857f5402a3577fe334ebfb3602ad0c2#f3ff9c1b48af14adf093e2af0c5cf2849829bb6d_186_248

We should only allow calling qt_target_qml_sources for real qml modules, aka targets that had qt_add_qml_module called on them.

We can relax this restriction in future Qt versions if the need arises.

This might imply documenting that in certain cases qml files should be added with qt_add_resources instead, but we need to very careful about advertising that if we do at all.