FormulaInstaller: make mode reader methods private

Library/Homebrew/formula_installer.rb

@@ -19,6 +19,7 @@ class FormulaInstaller
 
   def self.mode_attr_accessor(*names)
     attr_accessor(*names)
+    private(*names)
     names.each { |name| define_method("#{name}?") { !!send(name) }}
   end