Image types no longer report their chains. Instead, pipelines report
their packages and chains and blueprint packages are added to the
workload.
The distro.ImageType interface retains the PackageSetsChains() methods
for RHEL 7 until that is rewritten as well.
The osbuild-dnf-json-test doesn't use the PackageSetsChains() method
anymore. Instead, since it only test the centos-8 qcow2 image, it
hardcodes the expected package set names.
e4bb7f4f16