gather: Display source repo of packages
    
    When multiple repos are configured in pkgset_repos, the logs should
    contain information on where exactly the package was pulled from. The
    log file in question should be in
    `logs/<arch>/pkgset_source.<arch>.log`.
    
    Fixes: #545
    Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>