Add a plain `rhel-8` alias as the default distribution name and version
for the `rhel8` package. The `rhel-86` distro is still available via
the NewRHEL86() constructor. These two distributions are identical.
Repositories
------------
The rhel-8 repositories (repositories/rhel-8.json) are now set to the
CDN repositories with no minor version:
https://cdn.redhat.com/content/dist/rhel8/8/...
The rhel-8 test repositories (test/data/repositories/rhel-8.json) were
already set to the plain `8` repositories. The Google repos have been
added.
The test case generator repositories used for `rhel-8` are the rpmrepo
snapshots as for rhel-86.
cc5d32169d