Add note to not use PyCPS externally

Add a note that the PyCPS "cps" module is LGPL and therefore we don't
want to use it externally (accompanied by explicitly settings its
visibility). We don't expect that Drake would ever use it at other than
build time anyway.

tools/pycps.BUILD

@@ -9,6 +9,7 @@ py_library(
   name = "cps",
   srcs = ["cps.py"],
   deps = ["@semantic_version//:semantic_version"],
+  visibility = ["//visibility:private"], # LGPL; don't use externally
 )
 
 py_binary(