Create a separate module for our golangci-lint version (#80)

## Summary:
In principle, it's not a problem to have test-only deps in your go.mod,
because they won't end up in your importers' builds (and in newer Go
versions may not even be downloaded.  (Which is why there's no
annotation to do so.)  In practice, that doesn't really work for
golangci-lint, because it doesn't really use semver (reasonably, in that
any updated linter may break lint in your codebase).  And we don't
really need it other than to run the binary at a particular version.

So now, I put it in its own go module.  This requires a bit more
throat-clearing to run it (we can't just `go run`), but it's not so bad
and avoids anyone getting annoyed at us because we upgraded their
golangci-lint for them.

We could do the same for `internal/integration`, which adds quite a lot,
including gqlgen, to our dependency tree, but it's not clear there's a
need.

Fixes #62.

Issue: https://github.com/Khan/genqlient/issues/62

## Test plan:
make check


Author: benjaminjkraft

Reviewers: dnerdy, StevenACoffman, benjaminjkraft, aberkan, MiguelCastillo

Required Reviewers: 

Approved By: dnerdy, StevenACoffman

Checks: ✅ Test (1.17), ✅ Test (1.16), ✅ Test (1.15), ✅ Test (1.14), ✅ Test (1.13), ✅ Lint, ✅ Test (1.17), ✅ Test (1.16), ✅ Test (1.15), ✅ Test (1.14), ✅ Test (1.13), ✅ Lint

Pull Request URL: https://github.com/Khan/genqlient/pull/80

.github/workflows/go.yml

@@ -42,4 +42,4 @@ jobs:
     - name: Run lint
       uses: golangci/golangci-lint-action@v2
       with:
-        version: v1.42   # should match go.mod
+        version: v1.42   # should match internal/lint/go.mod