I had to sketch this out before I could get the internals of
systemd-sysupdate straight in my head, particularly around how an
`UpdateSet` points to one `Instance` from each of a set of `Resource`s,
and those `Instance`s are either all sources or all targets.
Hopefully this is useful to the next person to look at the code.
Signed-off-by: Philip Withnall <pwithnall@gnome.org>
