fix: Use /usr/lib/ rather than /lib in packaging code (#5970)

Support for unmerged /usr was removed from systemd a while ago:
https://lists.freedesktop.org/archives/systemd-devel/2022-September/048352.html

Furthermore, Debian tooling will now throw lintian errors if files
continue to reference /lib. Since it is no longer supported in upstream
systemd, there is no reason for it to be supported in upstream
cloud-init code that uses systemd.

setup_utils.py

@@ -16,11 +16,11 @@ def pkg_config_read(library: str, var: str) -> str:
     fallbacks = {
         "systemd": {
             "systemdsystemconfdir": "/etc/systemd/system",
-            "systemdsystemunitdir": "/lib/systemd/system",
-            "systemdsystemgeneratordir": "/lib/systemd/system-generators",
+            "systemdsystemunitdir": "/usr/lib/systemd/system",
+            "systemdsystemgeneratordir": "/usr/lib/systemd/system-generators",
         },
         "udev": {
-            "udevdir": "/lib/udev",
+            "udevdir": "/usr/lib/udev",
         },
     }
     cmd = ["pkg-config", f"--variable={var}", library]