Fix error rendering @ConvertProperty annotations during error reporting (#1648)

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.

stdlib/base.pkl

@@ -414,7 +414,7 @@ abstract class BaseValueRenderer {
 open class ConvertProperty extends Annotation {
   /// Function called by [BaseValueRenderer] types during rendering to transform property
   /// names and values.
-  render: (Pair<String, Any>, BaseValueRenderer) -> Pair<String, Any>
+  hidden render: (Pair<String, Any>, BaseValueRenderer) -> Pair<String, Any>
 }
 
 /// Base class for rendering Pkl values in some textual output format.