rojig ♲📦: Prepare server-side design for rojig-only

[cgwalters]

As part of rojig ♲📦 we want to support OS vendors shipping in "rojig only" mode, where they don't have a public OSTree repository; more strongly, support a mode where OSTree repositories are just optional caches.

Today,
rpm-ostree compose tree --ex-output-jigdo-set also requires --repo and things like the "did the commit change" detection logic rely on it.

What we want is something like: --ex-jigdo-repo that contains the input repository that contains jigdoRPMs - we then parse it server side and pull out the latest jigdoRPM, pull out the commit, and from there we have the "input checksum" we use to detect changes.

[cgwalters]

I started hacking on this, basically supporting drop the --repo argument to compose tree.

One thing I really quickly realized is that this basically requires depending on some sane way to version rpm-md repositories. In this mode, we're accepting an rpm-md repo as input and also our output is an rpm-md repo. Do we modify the input repository in place? Do we do something like what pungi does and output separate datestamped repos?

We might be able to punt on this a bit for now...basically mutate the input repo in place (require it to be file://). Anyone who wants to e.g. implement datestamped repos could do that by just CoW'ing the repo.

However, punting here defers the layered package problem; rpm-ostree deploy $version should also pin layered packages. Like yum distro-sync --version X. We could try adding just a little bit more metadata on top of pungi compose directories.

[cgwalters]

PR in #1512