gather: Log tag from which we pulled a package
    
    For each tag we ask Koji about (there might be more than one in
    modularity case), we create a log file with list of RPMs and details
    about which tag they were pulled from. This makes it easier to find out
    where the package is inherited from.
    
    Fixes: #547
    Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>