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/legacy_extension.go b/internal/impl/legacy_extension.go
index b845f35..bfa6e1b 100644
--- a/internal/impl/legacy_extension.go
+++ b/internal/impl/legacy_extension.go
@@ -54,8 +54,8 @@
 		// Create a new parent message and unwrap it if possible.
 		mv := mt.New().Interface()
 		t := reflect.TypeOf(mv)
-		if mv, ok := mv.(Unwrapper); ok {
-			t = reflect.TypeOf(mv.ProtoUnwrap())
+		if mv, ok := mv.(unwrapper); ok {
+			t = reflect.TypeOf(mv.protoUnwrap())
 		}
 
 		// Check whether the message implements the legacy v1 Message interface.