qt_internal_add_qml_module() calls through to qt6_add_plugin() but qt6_add_qml_module() doesn't

This difference means the internal function will do the following things, whereas the non-internal one won't:

• Add the arch suffix to the library file name on Android.
• Add the compiler defines QT_PLUGIN, QT_DEPRECATED_WARNINGS and QT_STATICPLUGIN (this last one only for static builds).

I'm assuming we want these things for qt6_add_qml_module() too. If so, then we should probably be calling qt6_add_plugin() internally inside qt6_add_qml_module(). If someone can confirm that, I can include the necessary change as part of my work on QTBUG-88763.