This replaces the copy of github.com/docker/docker/reference in the same
place, which we have just gotten rid of, and allows using this package
even in consumers which insist on an incompatible version of
docker/distribution.
The copy has been edited to drop a reference to
github.com/docker/distribution/digestset .
Signed-off-by: Miloslav Trmač <mitr@redhat.com>