Make `cask/macos` constants private.

Library/Homebrew/cask/macos.rb

@@ -238,6 +238,7 @@ module OS
                   .map(&method(:Pathname))
                   .to_set
                   .freeze
+    private_constant :SYSTEM_DIRS
 
     # TODO: There should be a way to specify a containing
     #       directory under which nothing can be deleted.
@@ -380,6 +381,7 @@ module OS
                         .to_set
                         .union(SYSTEM_DIRS)
                         .freeze
+    private_constant :UNDELETABLE_PATHS
 
     def system_dir?(dir)
       SYSTEM_DIRS.include?(Pathname.new(dir).expand_path)