Add simple depends_on API.

recipes/bottom.py

+from recipe_engine.recipe_api import Property
+from recipe_engine import config
+
+DEPS = [
+    'recipe_engine/properties',
+]
+
+PROPERTIES = {
+    'number': Property(kind=int),
+}
+
+RETURN_SCHEMA = config.ReturnSchema(
+    result=config.Single(int),
+)
+
+def RunSteps(api, number):
+  result = number * 2
+  return RETURN_SCHEMA(result=result)
+
+def GenTests(api):
+  yield (
+      api.test('basic') +
+      api.properties(number=3)
+  )