feat: ServicePackagesMixin, which packages a systemd service is (NetOrk/netork#1236)

get_service_packages() returns, per systemd service, the installed packages
it is made of: the one shipping its unit file and the one shipping the
binary its ExecStart runs -- not the libraries that binary links against.
One read-only command: systemctl show for every loaded service unit, then
one dpkg -S for all paths (each also across the /usr merge), or rpm -qf per
path. The report is framed; one cut short raises. A driver supplies
_run_service_packages_command, as for KernelFactsMixin.

Version 4.20.0.

README.md

@@ -123,6 +123,13 @@ everywhere, so the command and its parse are concrete here and a driver supplies
 `_run_kernel_facts_command`. `OSDriver` does not carry it — a Windows host is an OS driver
 too, and `hasattr(driver, "get_kernel_facts")` has to stay truthful.
 
+`ServicePackagesMixin` (`get_service_packages`) is mixed in the same way: which installed
+packages each systemd service is made of — the one shipping its unit file and the one
+shipping the binary its `ExecStart` runs, by `dpkg -S` or `rpm -qf`, not the libraries
+that binary links against. An application needs it to let a statement about a service
+(the postgresql unit) reach the findings on its packages. A driver supplies
+`_run_service_packages_command`.
+
 `HostStatusMixin` (`get_host_status`) is mixed in the same way: whether a Linux host needs
 a reboot to finish an update (`/var/run/reboot-required`, `needs-restarting -r`, or a newer
 kernel of the running flavour installed), whether it patches itself (unattended-upgrades,