gather: Ignore comps in lookaside repo
    
    This fixes a case where extra packages are pulled in.
    
    The scenario is:
    
     * there's a lookaside repo which contains group G which has package P
     * we want to pull group G into the compose, but our definition of G
       does not contain P
     * lookaside repo does not contain package P
     * current package set has P
    
    DNF depsolver will then merge the two definitions and try to get all the
    packages. For most cases this is not a problem, since the package is in
    the lookaside repo and will not be pulled into the compose. But in the
    example above since P is not in lookaside, Pungi will put it into
    current compose.
    
    This is also ugly in the depsolving log says it includes package P
    because it was in input. But checking comps file does not show it.
    
    The result of this change is that some packages might disappear from
    current composes. This can only happen if there is a lookaside which
    defines groups with similar IDs, which should be very rare. In cases
    where this would be a problem the fix is to explicitly add wanted
    packages either to comps or to additional packages.
    
    Fixes: https://pagure.io/pungi/issue/978
    Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>