gather: Always get latest packages
    
    If lookaside contains an older version of a package, but with a
    different arch, the depsolver doesn't notice that and prefers the
    lookaside version.
    
    This is not correct. The latest package should be used no matter if
    there are different arches available.
    
    The filtering in DNF doesn't ensure this, so we have to build it
    ourselves. To limit the performance impact, only run this filtering when
    there actually are some lookaside repos configured.
    
    JIRA: RHELCMP-11728
    
    Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>