gather-dnf: Run latest() later
    
    The initial version of the filtered the latest builds at the start. That
    doesn't matter in many cases:
    
    * When there are no lookaside repos, there is generally a single version
      of each package.
    * When lookaside repos do not overlap with compose repos, or contain
      only older versions.
    
    It is however a problem when the lookaside repos contain higher version
    of a package than what is in a compose repo, and some package explicitly
    requires the older version.
    
    Consider this scenario:
    
    * lookaside contains bar-1.1
    * compose repo contains bar-1.0 and foo-1.0
    * foo-1.0 `Requires: bar < 1.1`
    
    The original code would filter out the bar-1.0 package, and then fail on
    unresolved dependencies.
    
    This patch moves the computation of latest packages much later, to part
    of code where all options to satisfy a dependency are selected and the
    best match is chosen. At that point if there are multiple versions
    available, we do want the latest one.
    
    JIRA: SPMM-13483
    Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>