Bring back the PDC version of get_package_nvras, separately
    
    I've been mulling what to do about the whole 'getting NEVRAs
    from PDC' sitaution for a while. We *can* get package NEVRAs
    from PDC for composes that are in PDC, and when it works it's
    a lot cleaner than scraping them out of HTTP responses like we
    do in `get_package_nvras`, and it gets us more information
    (i.e. epochs).
    
    But obviously there are drawbacks. There's the time lag between
    a compose happening...then appearing in PDC...then the RPM data
    appearing in PDC (there are several minutes between each step
    in that chain). There's the fact that PDC doesn't have the best
    reliability record in the world (it just recently stopped
    importing *any* data for two weeks, for instance). And there's
    the issue that we can get epochs from PDC but we can't from
    HTTP scraping.
    
    In the end I've decided to go with this: bring back the PDC
    code, but as a *separate* method, which only Pungi4Release
    and subclasses have. This means you can choose either method for
    Pungi 4 releases, and there isn't the awkwardness of having the
    same method return not-fully-comparable information for Pungi 4
    and non-Pungi 4 composes.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>