Add Paths as first-class types in configs.

scripts/slave/recipe_modules/path/test_api.py

 from slave import recipe_test_api
+from slave.recipe_config_types import Path
 
 class PathTestApi(recipe_test_api.RecipeTestApi):
   @recipe_test_api.mod_test_data
   @staticmethod
   def exists(*paths):
+    assert all(isinstance(p, Path) for p in paths)
     return paths
+
+  def __getattr__(self, name):
+    return Path(name)
+