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/development/libraries/libusb1/default.nix

@@ -1,4 +1,4 @@
-{ stdenv, fetchurl, pkgconfig, libudev ? null, libobjc, IOKit }:
+{ stdenv, fetchurl, pkgconfig, systemd ? null, libobjc, IOKit }:
 
 stdenv.mkDerivation rec {
   name = "libusb-1.0.19";
@@ -12,13 +12,13 @@ stdenv.mkDerivation rec {
 
   buildInputs = [ pkgconfig ];
   propagatedBuildInputs =
-    stdenv.lib.optional stdenv.isLinux libudev ++
+    stdenv.lib.optional stdenv.isLinux systemd ++
     stdenv.lib.optionals stdenv.isDarwin [ libobjc IOKit ];
 
   NIX_LDFLAGS = stdenv.lib.optionalString stdenv.isLinux "-lgcc_s";
 
   preFixup = stdenv.lib.optionalString stdenv.isLinux ''
-    sed 's,-ludev,-L${libudev.out}/lib -ludev,' -i $out/lib/libusb-1.0.la
+    sed 's,-ludev,-L${systemd.lib}/lib -ludev,' -i $out/lib/libusb-1.0.la
   '';
 
   meta = {