[Go] Dependency pmezard/go-difflib is unmaintained

[asfimport]

I saw this error (transient?) in CI

Parsing meta tags from https://golang.org/x/xerrors?go-get=1 (status code 200)
get "golang.org/x/xerrors": found meta tag get.metaImport{Prefix:"golang.org/x/xerrors", VCS:"git", RepoRoot:"https://go.googlesource.com/xerrors"} at https://golang.org/x/xerrors?go-get=1
go: finding golang.org/x/xerrors v0.0.0-20191204190536-9bdfabe68543
go: github.com/pmezard/go-difflib@v1.0.0: unknown revision v1.0.0
go: error loading module requirements

Which led me to https://github.com/pmezard/go-difflib. It seems this package is not maintained. Should it be depended on?

Reporter: Wes McKinney / @wesm

_{Note: This issue was originally created as ARROW-8051. Please see the migration documentation for further details.}

[asfimport]

Matthew Topol / @zeroshade:
We only indirectly depend on go-difflib due to relying on github.com/stretchr/testify for the assert/mock libraries. github.com/stretchr/testify is maintained though, so I don't think this is a problem right now.