Required for running osbuild modules.
In Fedora, so far, python3 was always pulled by some other build-root
dependency. We should add it explicitly since it's required by all our
runners.
For RHEL and CentOS, the requirement is platform-python.
For RHEL 8 and CentOS 8, we also need to explicitly add python36 and for
RHEL 9 and CentOS 9 python3 because it's used to run the stages in
osbuild [1].
[1] ea8261cad6/runners/org.osbuild.rhel82 (L61)
For now all it does is represent the name of the runner and what requirements
it has of the build pipeline.
Move some package definitions from the runner package set to where it belongs.
