libimage: pull: don't resolve short names on explicit docker:// reference

[vrothberg]

… I might be missing something; how do short names like busybox vs explicit docker://docker.io/busybox pulls work? AFAICS this code parses both into a docker://busybox ImageReference, losing the distinction — and then may try to do short name lookup on explicit docker://… formats, which should, I think, never happen — short name lookup is used only for Docker-like non-transport-qualified formats, so that the c/image-transport-qualified strings always mean the same thing in all callers.

Originally posted by @mtrmac in #579 (comment)