Drop the rpm downloading and instead use the files source. This gives
us caching for free, and is the last missing step before we can
deprecate the dnf stage.
The main benefit of the rpm over the dnf stage is that we pin the package
versions rather than the repo metadata version. This will allow us to
support continuously changing repositories as individual packages are much
less likely to change than the repos iteself, and old packages are meant
to stay around for some time, unlike the repo metadata which is instantly
swapped out.
Depsolving is also slow on the first run, which we were always hitting as
the depsolving was always happening in a fresh container.
Based on a patch by Lars Karlitski.
Signed-off-by: Tom Gundersen <teg@jklm.no>
1d588b8e86