Add Paths as first-class types in configs.

scripts/slave/recipe_modules/generator_script/example.py

 # Copyright 2013 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 DEPS = [
   'generator_script',
   'path'
 ]
 
 
 def GenSteps(api):
-  api.path.add_checkout(api.path.slave_build())
+  api.path.set_dynamic_path('checkout', api.path.slave_build())
   yield api.generator_script('bogus')
   yield api.generator_script('bogus.py')
 
 
 def GenTests(api):
   yield (
     api.test('basic') +
     api.generator_script(
       'bogus',
       {'name': 'mock.step.binary', 'cmd': ['echo', 'mock step binary']}
     ) +
     api.generator_script(
       'bogus.py',
       {'name': 'mock.step.python', 'cmd': ['echo', 'mock step python']}
     )
   )