QML import-scanner runs at qmake time, not as extra compiler that depends on qml files

Being run at qmake time means any changes to a qml file to add a new import will not work until you do another qmake run. It would be preferable to run the import-scanner as an extra-compiler with proper makefile-deps, and only write the output-cpp file if the imports change, so that you dont get needless re-compilers of the import-cpp-file.