Fix check for DockerHub "library" reference
Fix check for DockerHub "library" reference

Check was incorrect due to target reference being created without a
hostname (after being split into hostname and repo:tag components to be
exact). This caused any non-repo delineated name to be associated with
the default DockerHub registry hostname and therefore caused the check
to incorrectly associate such names as being library references.

Signed-off-by: Phil Estes <dbac1f450dc8326caa7ba104b384a0eca642250c@gmail.com>

estesp/manifest-tool,estesp/manifest-tool