Use bundled ack instead of grep

Our ack is slightly modified to use `'\0'` characters instead of `':'` in the --nogroup listing. This makes it easier to handle both `':'` characters and newlines in filenames, but we aren't really doing that yet.

src/stdlib/require.coffee

@@ -92,6 +92,7 @@ resolve = (file) ->
   return file
 
 __expand = (path) ->
+  return path if __isFile path
   for ext, handler of exts
     if __exists "#{path}.#{ext}"
       return "#{path}.#{ext}"
@@ -104,6 +105,9 @@ __expand = (path) ->
 __exists = (path) ->
   $native.exists path
 
+__isFile = (path) ->
+  $native.isFile path
+
 __coffeeCache = (filePath) ->
   {CoffeeScript} = require 'coffee-script'
   tmpPath = "/tmp/atom-compiled-scripts"