Add tools package, tidy go mods

hack/tools.go

@@ -0,0 +1,9 @@
+// +build tools
+
+// This package imports things required by build scripts, to force `go mod` to see them as dependencies
+// https://github.com/golang/go/wiki/Modules#how-can-i-track-tool-dependencies-for-a-module
+package hack
+
+import (
+	_ "github.com/golangci/golangci-lint/cmd/golangci-lint"
+)