There should be a way to mark types exported to QML as "not exported"

The case of QtTest and Qt.test.qtestroot, as well as the "Templates" pattern used in qtquickcontrols2 shows that we have types that should be part of a module but not accessible to users of that module. Rather, they should be used only internally by the same module. This could be done with an extra C++ macro and an extra keyword in qmldir.