Formulary.core_path: use CoreFormulaRepository#formula_dir

Library/Homebrew/formulary.rb

@@ -326,7 +326,7 @@ class Formulary
   end
 
   def self.core_path(name)
-    Pathname.new("#{HOMEBREW_LIBRARY}/Formula/#{name.downcase}.rb")
+    CoreFormulaRepository.instance.formula_dir/"#{name.downcase}.rb"
   end
 
   def self.tap_paths(name, taps = Dir["#{HOMEBREW_LIBRARY}/Taps/*/*/"])