Now that tools' dependencies have been cleaned up in golang.org/cl/160837, lint
doesn't have any transitive dependencies on modules that import it using the
wrong path. It is now safe to reintroduce a go.mod file to this repo. I've
checked using dmitshur's instructions in
https://github.com/golang/lint/issues/436#issuecomment-469048301 that lint only
appears with its canonical module path in the build list.

Updates golang/lint#436

Change-Id: I6343aa103408b20562e17ea019602b159b899fc6
Reviewed-on: https://go-review.googlesource.com/c/lint/+/166278
Run-TryBot: Michael Matloob <matloob@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>