Our logic to export a resource as a stack output transforms the
resource into a plain old object by eliding internal fields and then
just serializing the resource as a POJO.
The custom serialization logic we used here unwrapped an Output
without care to see if it held a secret. Now, when it does, we
continue to return an Output as the thing to be serialized and that
output is marked as a secret.
Fixes#2862
