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

generate/types.go

@@ -281,8 +281,8 @@ func (builder *typeBuilder) writeType(name, namePrefix string, typ *ast.Type, fi
 	def := builder.schema.Types[typ.Name()]
 	goName, ok := builder.Config.Scalars[def.Name]
 	if ok {
-		name, err := builder.addRef(goName)
-		builder.WriteString(name)
+		newName, err := builder.addRef(goName)
+		builder.WriteString(newName)
 		return err
 	}
 	goName, ok = builtinTypes[def.Name]
@@ -303,7 +303,7 @@ func (builder *typeBuilder) writeTypedef(
 	typedef *ast.Definition,
 	pos *ast.Position,
 	fields []field,
-	options *GenqlientDirective,
+	options *GenqlientDirective, //nolint:unparam // it is used!
 	description string, // defaults to typedef.Description
 ) (err error) {
 	defer func() {