Move demandDirectoryRoot into kyaml/filesys (#4677)

* Move demandDirectoryRoot into kyaml/filesys

* Make root directory platform-agnostic

Support windows root directory. Dogfood own error package.

* Use cleaner windows support implementation

* Address feedback

* Address feedback x2

* Re-apply go.sum changes to avoid CI errors

kyaml/filesys/fsondisk_test_unix.go

@@ -0,0 +1,17 @@
+// Copyright 2022 The Kubernetes Authors.
+// SPDX-License-Identifier: Apache-2.0
+
+//go:build !windows
+// +build !windows
+
+package filesys
+
+import (
+	"path/filepath"
+	"testing"
+)
+
+func getOSRoot(t *testing.T) string {
+	t.Helper()
+	return string(filepath.Separator)
+}