hybrid: Get platform from lookaside repos
    
    If the hybrid solver is used in a situation where there are modules in
    lookaside repo, but not in the compose itself, it will fail to detect
    any platform. Since we are already opening the module repodata, we can
    retrieve platforms from all modules in there as well.
    
    If there are conflicts (e.g. multiple modules depending on different
    platforms), an error will be reported.
    
    JIRA: COMPOSE-3277
    Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>