Rename the QtWayland.Compositor import to QtWaylandCompositor

In Qt 6, we would like the QML import for the compositor API to be QtWaylandCompositor

The reason for this, is that we want to move specific funtionality into sub-modules i.e.

import QtWaylandCompositor 1.0
import QtWaylandCompositor.XdgShellV6 1.0
import QtWaylandCompositor.XdgShell 1.0

We should investigate if can start this process already in the 5.x series by supporting both imports in the same version, and deprecate the old one.