This moves libsystemd.so and libudev.so into systemd.lib, and gets rid
of libudev (which just contained a copy of libudev.so and the udev
headers). It thus reduces the closure size of all packages that
(indirectly) depend on libsystemd, of which there are quite a few (for
instance, PulseAudio and dbus). For example, it reduces the closure of
Blender from 430.8 to 400.8 MiB.
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