go mod

没有go.mod文件，导致依赖会有一些问题

``` go
github.com/google/go-cmp/cmp/cmpopts: github.com/sensorsdata/sa-sdk-go/v2@v2.1.0: reading https://goproxy.cn/github.com/sensorsdata/sa-sdk-go/v2/@v/v2.1.0.mod: 404 Not Found
server response: not found: github.com/sensorsdata/sa-sdk-go/v2@v2.1.0: invalid version: missing github.com/sensorsdata/sa-sdk-go/go.mod and .../v2/go.mod at revision v2.1.0
```