The added snippet test originally produced error "A value of type
`Function2` cannot be exported."
This PR actually fixes the bug twice:
* By marking `ConvertProperty.render` as `hidden` so that it is skipped
when the enclosing object is exported. This broke any attempts to obtain
the module schema because this requires exporting all annotations on all
class properties.
* By changing the way that `VmUndefinedValueException.fillInHint()`
obtains the module URI to avoid obtaining the module schema (and
triggering the more expensive module schema generation process).
It also makes function-typed annotation properties in `pkl:Command` hidden to avoid similar issues there.
Jen Basch