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/Command.pkl

@@ -165,7 +165,7 @@ class Flag extends BaseFlag {
   /// | [Listing], [List], [Set]   | Element values are parsed based on the above primitive types |
   /// | [Mapping], [Map], [Pair]   | Value is split into a [Pair] on the first `"="` and each substring is parsed based on the above primitive types |
   /// | Other types                | An error is thrown; `convert` should be defined explicitly |
-  convert: ((String) -> Any)?
+  hidden convert: ((String) -> Any)?
 
   /// Specifies whether the flag may be specified more than once.
   ///
@@ -188,7 +188,7 @@ class Flag extends BaseFlag {
   /// | [Listing], [List] | Result is all option values in the order specified |
   /// | [Set]             | Result is all unique option values |
   /// | Other types       | Result is the last value specified option value |
-  transformAll: ((List<Any>) -> Any)?
+  hidden transformAll: ((List<Any>) -> Any)?
 
   /// Specify how this flag should be completed in generated shell completions.
   ///
@@ -226,7 +226,7 @@ class Argument extends Annotation {
   ///
   /// If no value is provided, each option value is transformed using the same rules as
   /// [Flag.convert].
-  convert: ((String) -> Any)?
+  hidden convert: ((String) -> Any)?
 
   /// Specifies whether the argument may be specified more than once.
   ///
@@ -245,7 +245,7 @@ class Argument extends Annotation {
   ///
   /// If no value is provided, all option values are transformed using the same rules as
   /// [Flag.transformAll].
-  transformAll: ((List<Any>) -> Any)?
+  hidden transformAll: ((List<Any>) -> Any)?
 
   /// Specify how this flag should be completed in generated shell completions.
   ///

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.