Always add cask/lib to $LOAD_PATH.

Library/Homebrew/brew.rb

@@ -20,6 +20,10 @@ unless $LOAD_PATH.include?(HOMEBREW_LIBRARY_PATH.to_s)
   $LOAD_PATH.unshift(HOMEBREW_LIBRARY_PATH.to_s)
 end
 
+unless $LOAD_PATH.include?("#{HOMEBREW_LIBRARY_PATH}/cask/lib")
+  $LOAD_PATH.unshift("#{HOMEBREW_LIBRARY_PATH}/cask/lib")
+end
+
 require "global"
 
 begin

Library/Homebrew/cmd/cask.rb

@@ -1,4 +1,3 @@
-$LOAD_PATH.unshift("#{HOMEBREW_LIBRARY_PATH}/cask/lib")
 require "hbc"
 
 module Homebrew

Library/Homebrew/dev-cmd/irb.rb

@@ -47,7 +47,6 @@ module Homebrew
     require "formula"
     require "keg"
 
-    $LOAD_PATH.unshift("#{HOMEBREW_LIBRARY_PATH}/cask/lib")
     require "hbc"
 
     ohai "Interactive Homebrew Shell"

Library/Homebrew/test/spec_helper.rb

@@ -15,6 +15,7 @@ require "rubocop/rspec/support"
 require "find"
 
 $LOAD_PATH.unshift(File.expand_path("#{ENV["HOMEBREW_LIBRARY"]}/Homebrew"))
+$LOAD_PATH.unshift(File.expand_path("#{ENV["HOMEBREW_LIBRARY"]}/Homebrew/cask/lib"))
 $LOAD_PATH.unshift(File.expand_path("#{ENV["HOMEBREW_LIBRARY"]}/Homebrew/test/support/lib"))
 
 require "global"

Library/Homebrew/test/support/helper/spec/shared_context/homebrew_cask.rb

@@ -1,5 +1,3 @@
-$LOAD_PATH.push(HOMEBREW_LIBRARY_PATH.join("cask", "lib").to_s)
-
 require "hbc"
 
 require "test/support/helper/cask/fake_system_command"

Library/Homebrew/test/support/helper/spec/shared_context/integration_test.rb

@@ -85,6 +85,7 @@ RSpec.shared_context "integration test" do
         "-W0",
         "-I", "#{HOMEBREW_LIBRARY_PATH}/test/support/lib",
         "-I", HOMEBREW_LIBRARY_PATH.to_s,
+        "-I", "#{HOMEBREW_LIBRARY_PATH}/cask/lib",
         "-rconfig"
       ]
       if ENV["HOMEBREW_TESTS_COVERAGE"]