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_enum.go b/internal/impl/legacy_enum.go
index 4ec31df..c92eadf 100644
--- a/internal/impl/legacy_enum.go
+++ b/internal/impl/legacy_enum.go
@@ -101,7 +101,7 @@
 func (e *legacyEnumWrapper) ProtoReflect() pref.Enum {
 	return e
 }
-func (e *legacyEnumWrapper) ProtoUnwrap() interface{} {
+func (e *legacyEnumWrapper) protoUnwrap() interface{} {
 	v := reflect.New(e.goTyp).Elem()
 	v.SetInt(int64(e.num))
 	return v.Interface()
@@ -109,7 +109,7 @@
 
 var (
 	_ pref.Enum = (*legacyEnumWrapper)(nil)
-	_ Unwrapper = (*legacyEnumWrapper)(nil)
+	_ unwrapper = (*legacyEnumWrapper)(nil)
 )
 
 var legacyEnumDescCache sync.Map // map[reflect.Type]protoreflect.EnumDescriptor