Run golangci-lint in every Go project in repository (and fix all reported errors)

Created by: mrnugget

I noticed that golangci-lint only runs over the packages belonging to github.com/sourcegraph/sourcegraph. It doesn't run over lib, dev/sg, etc.

This PR changes the check script we use in CI to run golangci-lint over every project in our repository with a go.mod (which is what we do for go test in CI too).

That's the first commit. The second commit fixes all of the reported errors.

And since a lot of the reported errors were about dead code (that I deleted) I urge you to take a look, ask yourself whether you'll miss the dead code and if yes: hop on this branch here, bring the code back from the grave & make it undead so that golangci-lint doesn't complain anymore