Refactor and cleanup codes. No functional changes.

six.BUILD

@@ -0,0 +1,15 @@
+################################################################################
+# Copyright 2017 Google LLC.
+#
+# This software is licensed under the terms defined in the Widevine Master
+# License Agreement. For a copy of this agreement, please contact
+# widevine-licensing@google.com.
+################################################################################
+
+# Build target for six. Needed by py_proto_library.
+py_library(
+  name = "six",
+  srcs = ["six.py"],
+  srcs_version = "PY2AND3",
+  visibility = ["//visibility:public"],
+)