Remove unused imports and fix some other linter errors.

recipe_engine/loader.py

 # Copyright 2016 The LUCI Authors. All rights reserved.
 # Use of this source code is governed under the Apache License, Version 2.0
 # that can be found in the LICENSE file.
 
 import collections
-import contextlib
 import imp
 import inspect
 import os
 import sys
 
 from . import env
 
 from .config import ConfigContext, ConfigGroupSchema
 from .config_types import Path, ModuleBasePath, PackageRepoBasePath
 from .config_types import RecipeScriptBasePath
 from .config_types import RECIPE_MODULE_PREFIX
-from .recipe_api import RecipeApi, RecipeApiPlain, RecipeScriptApi
+from .recipe_api import RecipeApiPlain, RecipeScriptApi
 from .recipe_api import _UnresolvedRequirement
-from .recipe_api import Property, BoundProperty
+from .recipe_api import BoundProperty
 from .recipe_api import UndefinedPropertyException, PROPERTY_SENTINEL
 from .recipe_test_api import RecipeTestApi, DisabledTestData
 
 
 class LoaderError(Exception):
   """Raised when something goes wrong loading recipes or modules."""
 
 
 class NoSuchRecipe(LoaderError):
   """Raised by load_recipe is recipe is not found."""
@@ skipping 602 matching lines @@
   for k,v in toplevel_deps.iteritems():
     setattr(api, k, mapper.instantiate(v))
   return api
 
 
 def _resolve_requirement(req, engine):
   if req._typ == 'client':
     return engine._get_client(req._name)
   else:
     raise ValueError('Unknown requirement type [%s]' % (req._typ,))