This finally uses all of the ImageSource and ImageDestination
implementations, though these utilities are in turn not used yet.
Adds unresolved FIXME (FIXME!!) notes for the tlsVerify default value;
for now, the code follows the existing parseImage semantics.
Also note the naming inconsistency: dir:…, atomic:…, but
docker://… . I think the non-// names are cleaner, but if we are
committed to docker://…, just being consistent might be better.
