systemd and udev have been merged upstream. We will still ship them in separate packages. However, in order to keep things simple, udev will now be part of a package called systemd-tools. This package contains several other standalone tools which can be used without systemd. The astute reader will note that this also means the entirety of systemd is available in the core repository.
Please replace udev with systemd-tools when prompted. If you upgrade the linux-libre package at the same time, you may see an error during initramfs creation that the udev hook is not found. After the upgrade completes, please rerun 'mkinitcpio -p linux-libre' to ensure that a bootable image is created for the newly installed kernel.