qmlplugindump ignores base classes of QML files registered as components

As can be seen in https://codereview.qt-project.org/c/qt/qtvirtualkeyboard/+/275021 and https://codereview.qt-project.org/c/qt/qtquickcontrols/+/275025 qmlplugindump considers all components derived from Item as having the prototype "QObject" and repeats all the properties from QQuickItem in each of them.