Fix loading field message when dependency is more than one step (#54)

Fixes the problem where you would get 
```
compilation error: failed to compile expression api.source_context.file_name: ERROR: <input>:1:4: unexpected failed resolution of 'google.protobuf.SourceContext'
                                 | has(this.source_context.file_name)
                                 | ...^
``` 
for the example added.

internal/evaluator/builder.go

@@ -268,7 +268,7 @@ func (bldr *Builder) processFieldExpressions(
 	var opts []cel.EnvOption
 	if fieldDesc.Kind() == protoreflect.MessageKind {
 		opts = []cel.EnvOption{
-			cel.TypeDescs(fieldDesc.Message().ParentFile()),
+			cel.Types(dynamicpb.NewMessage(fieldDesc.Message())),
 			cel.Variable("this", cel.ObjectType(string(fieldDesc.Message().FullName()))),
 		}
 	} else {