treewide: add license header and enable haslicense linter

Change-Id: I873a8d4082d75e8f813d8a726a41187eea7a065e
Reviewed-on: https://review.monogon.dev/c/monogon/+/3825
Tested-by: Jenkins CI
Reviewed-by: Leopold Schabel <leo@monogon.tech>
diff --git a/go/algorithm/cartesian/cartesian.go b/go/algorithm/cartesian/cartesian.go
index fb42a2b..54079b8 100644
--- a/go/algorithm/cartesian/cartesian.go
+++ b/go/algorithm/cartesian/cartesian.go
@@ -1,3 +1,6 @@
+// Copyright The Monogon Project Authors.
+// SPDX-License-Identifier: Apache-2.0
+
 package cartesian
 
 // Product returns cartesian product of arguments. Each argument must be a slice
diff --git a/go/algorithm/cartesian/cartesian_test.go b/go/algorithm/cartesian/cartesian_test.go
index 5911ecc..9834483 100644
--- a/go/algorithm/cartesian/cartesian_test.go
+++ b/go/algorithm/cartesian/cartesian_test.go
@@ -1,3 +1,6 @@
+// Copyright The Monogon Project Authors.
+// SPDX-License-Identifier: Apache-2.0
+
 package cartesian
 
 import (