depot_tools: add infra_paths recipe module for infra-specific paths

recipe_modules/rietveld/example.py

Index: recipe_modules/rietveld/example.py
diff --git a/recipe_modules/rietveld/example.py b/recipe_modules/rietveld/example.py
index b00cb201a34c580bf2294e2fa319582327e0b72a..b389687c560cd3871255e841b8b3270dda1742ca 100644
--- a/recipe_modules/rietveld/example.py
+++ b/recipe_modules/rietveld/example.py
@@ -3,6 +3,7 @@
 # found in the LICENSE file.
 
 DEPS = [
+  'infra_paths',
   'recipe_engine/path',
   'recipe_engine/properties',
   'recipe_engine/step',
@@ -10,7 +11,7 @@ DEPS = [
 ]
 
 def RunSteps(api):
-  api.path['checkout'] = api.path['slave_build']
+  api.path['checkout'] = api.infra_paths['slave_build']
   api.rietveld.apply_issue('foo', 'bar', authentication='oauth2')
   api.rietveld.calculate_issue_root({'project': ['']})