support standard go mod #1188

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. What did you do?
   go get github.com/pingcap/parser/v4

2. What did you expect to see?

no error

3. What did you see instead?

go get: module github.com/pingcap/parser@upgrade found (v0.0.0-20200623164729-3a18f1e5dceb), but does not contain package github.com/pingcap/parser/v4

4. What version of TiDB SQL Parser are you using?
   v4.0.2