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/api_export.go b/internal/impl/api_export.go
index 55c5481..f2b3738 100644
--- a/internal/impl/api_export.go
+++ b/internal/impl/api_export.go
@@ -74,8 +74,8 @@
 	switch mv := m.(type) {
 	case piface.MessageV1:
 		return mv
-	case Unwrapper:
-		return Export{}.ProtoMessageV1Of(mv.ProtoUnwrap())
+	case unwrapper:
+		return Export{}.ProtoMessageV1Of(mv.protoUnwrap())
 	case protoreflect.ProtoMessage:
 		return legacyMessageWrapper{mv}
 	default: