formulary: use CoreFormulaRepository

Let's hide implementation detail on where to find core formulae alias
directory. This will benefit future core code and formulae separation.

Library/Homebrew/formulary.rb

@@ -276,7 +276,7 @@ class Formulary
       return FormulaLoader.new(ref, formula_with_that_name)
     end
 
-    possible_alias = Pathname.new("#{HOMEBREW_LIBRARY}/Aliases/#{ref}")
+    possible_alias = CoreFormulaRepository.instance.alias_dir/ref
     if possible_alias.file?
       return AliasLoader.new(possible_alias)
     end

Library/Homebrew/test/test_formulary.rb

@@ -69,7 +69,7 @@ class FormularyFactoryTest < Homebrew::TestCase
   end
 
   def test_factory_from_alias
-    alias_dir = HOMEBREW_LIBRARY/"Aliases"
+    alias_dir = CoreFormulaRepository.instance.alias_dir
     alias_dir.mkpath
     FileUtils.ln_s @path, alias_dir/"foo"
     assert_kind_of Formula, Formulary.factory("foo")