We need an "internal import" directive in qmldir that imports the specified module into the implicit import of all its QML files, does not re-export its types. At least that's the current idea. If not feasible some other solution has to be found.
With such a thing, we could add "internal import QtQuick.Templates/auto" to each of the styles' qmldir files and in turn drop the imports from the individual .qml files. This way templates would be imported with the same version as Controls.