don't use new name rack if old name rack is a dir

Everything that used HOMEBREW_CELLAR/canonical_name
could point to something that doesn't exist because
loader_for tries to load new name formula if no old
name found. However there can be software installed
from path with the same name that renamed formulae
had and we still need to link/unlink/uninstall etc
that software. The solution is Formulary#to_rack
method that returns rack for given name.
- Add Formulary#to_rack
- Update ARGV.kegs
- Update cmd/switch

Library/Homebrew/cmd/switch.rb

@@ -12,8 +12,7 @@ module Homebrew
     name = ARGV.shift
     version = ARGV.shift
 
-    canonical_name = Formulary.canonical_name(name)
+    rack = Formulary.to_rack(name)
-    rack = HOMEBREW_CELLAR.join(canonical_name)
 
     unless rack.directory?
       onoe "#{name} not found in the Cellar."

Library/Homebrew/extend/ARGV.rb

@@ -47,14 +47,14 @@ module HomebrewArgvExtension
     require "keg"
     require "formula"
     @kegs ||= downcased_unique_named.collect do |name|
-      canonical_name = Formulary.canonical_name(name)
+      rack = Formulary.to_rack(name)
-      rack = HOMEBREW_CELLAR/canonical_name
+
       dirs = rack.directory? ? rack.subdirs : []
 
-      raise NoSuchKegError.new(canonical_name) if dirs.empty?
+      raise NoSuchKegError.new(rack.basename) if dirs.empty?
 
-      linked_keg_ref = HOMEBREW_LIBRARY.join("LinkedKegs", canonical_name)
+      linked_keg_ref = HOMEBREW_LIBRARY.join("LinkedKegs", rack.basename)
-      opt_prefix = HOMEBREW_PREFIX.join("opt", canonical_name)
+      opt_prefix = HOMEBREW_PREFIX.join("opt", rack.basename)
 
       begin
         if opt_prefix.symlink? && opt_prefix.directory?
@@ -66,7 +66,7 @@ module HomebrewArgvExtension
         elsif (prefix = (name.include?("/") ? Formulary.factory(name) : Formulary.from_rack(rack)).prefix).directory?
           Keg.new(prefix)
         else
-          raise MultipleVersionsInstalledError.new(canonical_name)
+          raise MultipleVersionsInstalledError.new(rack.basename)
         end
       rescue FormulaUnavailableError
         raise <<-EOS.undent

Library/Homebrew/formulary.rb

@@ -195,6 +195,24 @@ class Formulary
     end
   end
 
+  def self.to_rack(ref)
+    name = canonical_name(ref)
+    rack = HOMEBREW_CELLAR/name
+
+    # Handle the case when ref is an old name and the installation
+    # hasn't been migrated or when it's a package installed from
+    # path but same name formula was renamed.
+    unless rack.directory?
+      if ref =~ HOMEBREW_TAP_FORMULA_REGEX
+        rack = HOMEBREW_CELLAR/$3
+      elsif !ref.include?("/")
+        rack = HOMEBREW_CELLAR/ref
+      end
+    end
+
+    rack
+  end
+
   def self.canonical_name(ref)
     loader_for(ref).name
   rescue TapFormulaAmbiguityError