Add rpm_to_build() method to koji_utils
    
    This method is required for depcheck's squashing of per-rpm results
    to per-update results.
    It takes a rpm filename on input and returns build info from koji.
    It has "caching/prefetching" capabilities to speed-up subsequent queries.
    
    Differential Revision: https://phab.qadevel.cloud.fedoraproject.org/D148