No need to use layers to find parent image
    
    find_parent_images_with_package was still called using layers as list
    of images. But `layers` is a list of string, which is not what we
    expect. For this reason an error is raised: 'str' object does not
    support item assignment.
    
    Signed-off-by: Giulia Naponiello <gnaponie@redhat.com>