switch to go vendoring

vendor/github.com/google/go-cmp/cmp/export_panic.go

@@ -0,0 +1,16 @@
+// Copyright 2017, The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build purego
+// +build purego
+
+package cmp
+
+import "reflect"
+
+const supportExporters = false
+
+func retrieveUnexportedField(reflect.Value, reflect.StructField, bool) reflect.Value {
+	panic("no support for forcibly accessing unexported fields")
+}