hybrid: Download remote files when getting platform
    
    When probing lookasides for platform definition, we need to make sure it
    works for repos specified as HTTP urls. Createrepo doesn't seem to
    automatically download the repodata, so we have to help it.
    
    JIRA: COMPOSE-3958
    Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>