Show error if qt_add_qml_module is called twice in the same directory scope

Context: https://codereview.qt-project.org/c/qt/qtdeclarative/+/359918/6#message-7f1fe70b81848585757ac126b17c039c4fd7dd83

One might try to add 2 qml modules in the same directory scope like here
https://codereview.qt-project.org/c/qt/qtdeclarative/+/359918/5/tests/auto/qml/qmlbasicapp/CMakeLists.txt

This would silently override the qmldir.

We should FATAL_ERROR if qt_add_qml_module is called twice within the same directory scope.

Perhaps also if qt_add_qml_plugin is called twice in the same scope, it that even make sense.