Improve fs package and doc in prep to officially go public

pkg/fs/rpath.go

@@ -0,0 +1,12 @@
+// Copyright 2019 The Kubernetes Authors.
+// SPDX-License-Identifier: Apache-2.0
+
+package fs
+
+import "path/filepath"
+
+// RPath returns a rooted path, e.g. "/hey/foo" as
+// opposed to "hey/foo".
+func RPath(elem ...string) string {
+	return separator + filepath.Join(elem...)
+}