Extend FileUtils rather than include it

Fixes Homebrew/homebrew#10729.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>

Library/Homebrew/extend/fileutils.rb

@@ -1,8 +1,7 @@
 require 'fileutils'
 
 # We enhance FileUtils to make our Formula code more readable.
-module Homebrew::FileUtils
-  include FileUtils
+module FileUtils extend self
 
   # Create a temporary directory then yield. When the block returns,
   # recursively delete the temporary directory.
@@ -29,7 +28,7 @@ module Homebrew::FileUtils
 
   # A version of mkdir that also changes to that folder in a block.
   def mkdir name, &block
-    super(name)
+    FileUtils.mkdir(name)
     if block_given?
       chdir name do
         yield

Library/Homebrew/formula.rb

@@ -6,7 +6,7 @@ require 'extend/fileutils'
 
 # Derive and define at least @url, see Library/Formula for examples
 class Formula
-  include Homebrew::FileUtils
+  include FileUtils
 
   attr_reader :name, :path, :url, :version, :homepage, :specs, :downloader
   attr_reader :standard, :unstable