prepare: Return full package names

Previously we were truncating a list like

    0:glibc-devel-2.12-1.132.el6.i686
    0:glibc-devel-2.12-1.132.el6.x86_64

to be

    glibc-devel
    glibc-devel

wich gets uniqified. So we weren't noticing packages for multiple
architectures. Yum can handle the full names, so just return those.