Push users to use the DEPENDS option in qt_add_qml_module

If you don't, you manage your dependencies via import statements in QML files. qmlimportscanner hast to read those, and you have to manually run CMake everytime you add or remove a dependency. This is brittle.

The DEPENDS option states the dependencies of a module in a much clearer way and allows qmllint's and qmlcachegen's type inference to do a better job. In order to add or remove a dependency there, you edit the CMakeLists.txt file which automatically triggers a re-run of CMake.