QtRemoteObjects CMake API is using the internal qt_manual_moc API

Discovered as part of review at https://codereview.qt-project.org/c/qt/qtremoteobjects/+/364791/4#message-ad5271abda222f469147a85adc84e6004bdfd25a

Both _qt_internal_add_repc_files and qt6_reps_from_headers use qt_manual_moc which is private CMake api defined in qtbase/cmake/QtAutogenHelpers.cmake

It's not clear if the API works at all.

It should either be replaced by using AUTOMOC or _qt_internal_create_moc_command / qt6_generate_moc.

One particular bump in doing in that is that the RemoteObjects API seems to rely on the OUTPUT_MOC_JSON_FILES parameter of qt_manual_moc, which is not exposed by any of the public equivalents afaik.