Wayland extension XML file unnecessarily needs to match protocol

Auto generating sources for a Wayland extension from an XML file (e.g. with qt6_generate_wayland_protocol_server_sources) requires the file name to match the protocol name within the XML code. The reason is that auto generated file names are derived from the XML file name, but the actual code (like #include files) is derived from the protocol name. This restriction is unnecessary. The problem exists with cmake, as well as with qmake.