Instead of having a static repository checksum, set it dynamically from
the metadata that osbuild-composer last saw. This is implemented in
dnf-json, which returns the checksums for each repository on every call.
This enables the use of repositories that change over time, such as
fedora-updates. Note that the osbuild pipeline will break when such a
repository changes. This is intentional: pipelines have to be
reproducible.
RHEL and Fedora used different output formats (.ami vs .raw.xz). The job
package assumed `image.ami`, which failed for RHEL.
Change Fedora to use image.raw.xz as well. This makes it consistent, but
we should also get the filename from the distro at some point.
Automatically registering on `init()` is clever, but a bit too magical
and easy to get wrong, because every binary must include all distros
somewhere.
Flip this inside out: distros now have a `New()`, which returns
something that implements the `Distro` interface. The distro package
explicitly creates all of them.
This means that distros cannot import distro itself anymore, because go
forbids import cycles. This only affected `InvalidOutputFormatError`.
Return a generic error for now.
