tap: add formula_renames

- Tap#formula_renames is a method used for getting information
about renames in tap formulae.

Library/Homebrew/tap.rb

@@ -1,3 +1,5 @@
+require "utils/json"
+
 # a {Tap} is used to extend the formulae provided by Homebrew core.
 # Usually, it's synced with a remote git repository. And it's likely
 # a Github repository with the name of `user/homebrew-repo`. In such
@@ -137,6 +139,15 @@ class Tap
     }
   end
 
+  # Hash with tap formula renames
+  def formula_renames
+    @formula_renames ||= if (rename_file = path/"formula_renames.json").file?
+      Utils::JSON.load(rename_file.read)
+    else
+      {}
+    end
+  end
+
   def self.each
     return unless TAP_DIRECTORY.directory?