Support source generation #48
Description
We recently merged Rust code into rpm-ostree. Previously, rpm-ostree was buildable with rdgo. It isn't with --enable-rust because cargo wants to fetch bits from the internets.
COPR today has a "Custom" AKA make srpm path which generates sources. I think we should do something similar although I'd say we provide a git repository by default, and the process should make one or more commits on top of that. We could also equivalently have the code generate a tarball, and then unpack+commit that, but I like encouraging git.
Concretely for rpm-ostree today though we'd probably use the cargo vendor bits which are part of our automake-based tarball generation.