internal/impl: unexport Unwrapper

This interface is only ever used within the impl package.
Unexport it to avoid potential abuse by users.

Change-Id: I58d0cc2e8cb01c8c9d36cdec469c8d2196c4f836
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/193197
Reviewed-by: Damien Neil <dneil@google.com>
diff --git a/internal/impl/message_reflect_field.go b/internal/impl/message_reflect_field.go
index 6f7051b..e55b6de 100644
--- a/internal/impl/message_reflect_field.go
+++ b/internal/impl/message_reflect_field.go
@@ -189,7 +189,7 @@
 		},
 		set: func(p pointer, v pref.Value) {
 			rv := p.Apply(fieldOffset).AsValueOf(fs.Type).Elem()
-			rv.Set(reflect.ValueOf(v.List().(Unwrapper).ProtoUnwrap()).Elem())
+			rv.Set(reflect.ValueOf(v.List().(unwrapper).protoUnwrap()).Elem())
 		},
 		mutable: func(p pointer) pref.Value {
 			v := p.Apply(fieldOffset).AsValueOf(fs.Type)