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/convert_map.go b/internal/impl/convert_map.go
index ca51819..68fb506 100644
--- a/internal/impl/convert_map.go
+++ b/internal/impl/convert_map.go
@@ -105,6 +105,6 @@
 func (ms *mapReflect) NewValue() pref.Value {
 	return ms.valConv.New()
 }
-func (ms *mapReflect) ProtoUnwrap() interface{} {
+func (ms *mapReflect) protoUnwrap() interface{} {
 	return ms.v.Interface()
 }