Add chefstyle library file for Rake backwards compatibility

There's a decent amount of Rake code that uses `require "chefstyle"` to set the
defaults for RuboCop (eg
https://github.com/chef/ohai/blob/6d64237f9987c3bf51805e19884e6e710c3a40f6/Rakefile#L19),
and rather than doing surgery on each repo's Rakefile, it'd be easier to keep
that same behavior for loading the configuration and cops. With this change
a migration involving Rake should only involve a Gemfile update and changing
`require "chefstyle"` to `require "cookstyle/chefstyle"`.

Signed-off-by: David Crosby <[email protected]>

bin/cookstyle

@@ -1,16 +1,15 @@
 #!/usr/bin/env ruby
 # frozen_string_literal: true
 
-if ARGV.include?('--chefstyle')
-  ARGV.delete('--chefstyle')
-  module Cookstyle
-    CHEFSTYLE_CONFIG = true
-  end
-end
 
 $LOAD_PATH.unshift File.join(__dir__, %w(.. lib))
 
-require 'cookstyle'
+if ARGV.include?('--chefstyle')
+  ARGV.delete('--chefstyle')
+  require 'cookstyle/chefstyle'
+else
+  require 'cookstyle'
+end
 
 # force the fail level to :convention so that we can set all our new rules to
 # the lowest level of :refactor without failing everyone's CI jobs

lib/cookstyle/chefstyle.rb

@@ -0,0 +1,4 @@
+module Cookstyle
+  CHEFSTYLE_CONFIG = true
+end
+require_relative '../cookstyle'