Split copier.copyOneImage from copy.Image
Split copier.copyOneImage from copy.Image

After building all that infrastructure, things start to fall into place:
separate the code common to handling individual images and manifest lists from
the code to copy a single image.

This only extracts a method, does not change behavior.

Signed-off-by: Miloslav Trmač <9c008e38982a5397deb855345fb164f0558459ae@redhat.com>

containers/image,containers/image