Restore return type of BaseModule.toTarget to ModuleTarget (#2117)

Definition/Instance introduced the need for representing the targets of
instances as InstanceTargets. This original implementation changed the
return type of BaseModule.toTarget to express this need. This is a
backwards incompatible change that is actually unnecessary because it is
impossible for users to get references to the internal InstanceClone
objects, instead only accessing such modules via Instance[_] wrappers
and cloned Data. We restored the old API by adding a new internal method
"getTarget" which will give the correct targets for InstanceClones while
maintaining the API of BaseModule.toTarget.