Move static plugin object_init code from qt_add_qml_module to qt_add_qml_plugin

Currently we call __qt_internal_add_static_plugin_init_object_library and __qt_internal_propagate_object_library for a static qml plugin inside the implementation of qt_add_qml_module.

https://codereview.qt-project.org/c/qt/qtdeclarative/+/373606/1/src/qml/Qt6QmlMacros.cmake#589

Arguably it should be moved to the implementation of qt_add_qml_plugin instead.

A concern was raised about a corner case here

if the user passes NO_CREATE_PLUGIN_TARGET, the backing target and plugin target are the same, and they pass in an existing target, then there is a code path to avoid calling qt6_add_qml_plugin().

https://codereview.qt-project.org/c/qt/qtdeclarative/+/372860/4/src/qml/Qt6QmlMacros.cmake#601