support mixing noarch and archful packages in a yum repo build
    
    Currently the YumRepoBuild class assumes that *every* package you pass
    to it was built for the requested architecture. But this makes it
    impossible to put noarch packages into the repository, because
    YumRepoBuild will look for an arch-specific package when only a noarch
    package was produced.
    
    Change YumRepoBuild to only copy over packages if they actually produced
    the requested arch, otherwise skip them.