systemd: Separate lib output

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.

pkgs/os-specific/linux/util-linux/default.nix

@@ -20,7 +20,6 @@ stdenv.mkDerivation rec {
   # due to lots of ${utillinux}/bin occurences and headers being rather small
   outputDev = "bin";
 
-
   #FIXME: make it also work on non-nixos?
   postPatch = ''
     # Substituting store paths would create a circular dependency on systemd
@@ -57,7 +56,7 @@ stdenv.mkDerivation rec {
   buildInputs =
     [ zlib pam ]
     ++ lib.optional (ncurses != null) ncurses
-    ++ lib.optional (systemd != null) [ systemd pkgconfig ]
+    ++ lib.optional (systemd != null) systemd
     ++ lib.optional (perl != null) perl;
 
   postInstall = ''