Go mod vendor

Signed-off-by: Shengjing Zhu <zhsj@debian.org>

vendor/golang.org/x/xerrors/doc.go

@@ -0,0 +1,22 @@
+// Copyright 2019 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.
+
+// Package xerrors implements functions to manipulate errors.
+//
+// This package is based on the Go 2 proposal for error values:
+//   https://golang.org/design/29934-error-values
+//
+// These functions were incorporated into the standard library's errors package
+// in Go 1.13:
+// - Is
+// - As
+// - Unwrap
+//
+// Also, Errorf's %w verb was incorporated into fmt.Errorf.
+//
+// Use this package to get equivalent behavior in all supported Go versions.
+//
+// No other features of this package were included in Go 1.13, and at present
+// there are no plans to include any of them.
+package xerrors // import "golang.org/x/xerrors"