Create proper wrapper scripts to decouple from swarm_client.

scripts/slave/swarming/swarming_utils.py

 #!/usr/bin/env python
 # Copyright 2013 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 """Code to find swarming_client."""
 
 import os
 
 
 def find_client(base_dir):
-  """Returns the path to swarming_client if found."""
+  """Returns the path to swarming_client if found.
+
+  |base_dir| will be in general os.getcwd(), so the script is very dependent on
+  CWD. CWD should be the base directory of the checkout. It has always been the
+  case.
+  """
   src_swarming_client = os.path.join(
       base_dir, 'src', 'tools', 'swarming_client')
   if os.path.isdir(src_swarming_client):
     return src_swarming_client
 
   # This is the previous path. This can be removed around 2013-12-01.
   src_swarm_client = os.path.join(base_dir, 'src', 'tools', 'swarm_client')
   if os.path.isdir(src_swarm_client):
     return src_swarm_client