Don't put the plugins in a "/plugins" sub-dir in the builddir
This restores the old behaviour of putting the .so files in builddir/bin/<plugin_name_space>, where plugin_name_space is e.g. "kf5/kio/".
This restores the old behaviour of putting the .so files in builddir/bin/<plugin_name_space>, where plugin_name_space is e.g. "kf5/kio/".