FedoraMessaging: include NVRs in update messages, change id
    
    This tweaks the FedoraMessaging plugin to include a 'builds'
    array in the artifact hash for update messages, including a
    single-key hash for each build in the update, as required for
    the 'fedora-update' artifact by the spec (this was a request
    from the CI Messages maintainers). It also changes the 'id'
    for the artifact in update messages to be an sha256 hash of
    the NVRs and include the update alias as 'alias' rather than
    'id' (again as required by the spec).