Add Paths as first-class types in configs.

scripts/slave/recipe_modules/path/test_api.py

Index: scripts/slave/recipe_modules/path/test_api.py
diff --git a/scripts/slave/recipe_modules/path/test_api.py b/scripts/slave/recipe_modules/path/test_api.py
index 6834fa399ba5d919412548fa2c63fd12cecc5932..887dcad8c3e9cd6df40d17af40ec5e1f9ba41b31 100644
--- a/scripts/slave/recipe_modules/path/test_api.py
+++ b/scripts/slave/recipe_modules/path/test_api.py
@@ -1,7 +1,13 @@
 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, bypass=True)
+