utils: replace shell which with native code

Originally written for tigerbrew, but useful enough for core.
Replaces the shelled-out which in utils.rb with a native-ruby
equivalent, which is moderately faster.

Closes Homebrew/homebrew#16659.

Signed-off-by: Misty De Meo <mistydemeo@gmail.com>

Library/Homebrew/utils.rb

@@ -152,12 +152,8 @@ def puts_columns items, star_items=[]
 end
 
 def which cmd
-  path = `/usr/bin/which #{cmd} 2>/dev/null`.chomp
-  if path.empty?
-    nil
-  else
-    Pathname.new(path)
-  end
+  dir = ENV['PATH'].split(':').find {|p| File.executable? File.join(p, cmd)}
+  Pathname.new(File.join(dir, cmd)) unless dir.nil?
 end
 
 def which_editor