Enable golangci-lint (#49)

## Summary:
It would be nice to have some linting beyond `go vet`!  Now we do.  I
started by copying the config from Khan/webapp.  I did remove a couple
of staticcheck checks that I didn't feel were useful.  (Note also that
exportloopref is the replacement for scopelint in newer golangci-lint.)

Included are all the needed lint fixes; most are stylistic but the
changes in the example are a (minor) bugfix.

Fixes #22.
Issue: https://github.com/Khan/genqlient/issues/22

## Test plan:
make check

Author: benjaminjkraft

Reviewers: aberkan, dnerdy, benjaminjkraft, csilvers, MiguelCastillo

Required Reviewers: 

Approved by: aberkan, dnerdy

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/49

internal/testutil/tools.go

@@ -0,0 +1,13 @@
+// Conventionally this sort of thing would use the "ignore" tag, but
+// `go mod tidy` ignores so-tagged files explicitly, so we use another build
+// tag we never intend to set.
+// +build tools
+//go:build tools
+
+package testutil
+
+import (
+	// Keep golangci-lint from getting pruned from the go.mod.  We need it in
+	// go.mod so that we can easily `go run` it in `make check`.
+	_ "github.com/golangci/golangci-lint/cmd/golangci-lint"
+)