coreos-koji-tagger: adjust strategy to handle rpm names as input
    
    Previously I was considering the input from the lockfile to be
    names of koji builds rather than individual rpm names, which could
    actually be names of subpackages.
    
    Also added a comment section describing overall strategy and nomenclature
    (i.e. what terminology means in koji and throughout this script), which
    should help future readers.
    
    Added a few helper functions to aid in the new strategy.
        - get_srpms_from_rpmnvras
        - def get_builds_from_rpmnevras