Move org.kde.plasma.private.sessions module elsewhere, so other projects won't run into dependency cycles
Currently KScreenLocker requires plasma-workspace as a runtime-only dependency for CI, otherwise tests would fail. However, plasma-workspace is a chunky dependency which in turn also requires KWin etc. so we'd rather avoid that. KScreenLocker only needs it for the org.kde.plasma.private.sessions
QML module; but last time I checked that module unfortunately required PW::KWorkspace library on its own which isn't ideal.