Drop Obsoletes for systemd-standalone-{tmpfiles,sysusers}
It turns out that with the Obsoletes, dnf will just install the normal
systemd package if systemd-standalone-* is requested. The commit message
for b36512ad8f06270739f85371d03f61bbf8e6fe2a which added this says I tested
with local package builds (where it works), but not when going through the
full repo with all packages.
I'm adding the Provides instead, so that it's possible to request on or
the other more easily.