/usr/lib/x86_64-linux-gnu/qt5/qml/QtQuick/Dialogs/Private/qmldir is in qml-module-qtquick-dialogs 5.9.5-0ubuntu1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 | module QtQuick.Dialogs.Private
plugin dialogsprivateplugin
classname QtQuick2DialogsPrivatePlugin
typeinfo plugins.qmltypes
|