Clean up imports

manifest_xml: import `HEAD` and `R_HEADS` from correct module
version: import `HEAD` from correct module

`HEAD` and `R_HEADS` should be imported from the git_refs module,
where they are originally defined, rather than from the project
module.

repo: remove unused import of readline

cherry_pick: import standard modules on separate lines
smartsync: import subcmd modules explicitly from subcmd

Use:
  `import re
  import sys`
and
  `from subcmds.sync import Sync`

Instead of:
  `import sys, re`
and
  `from sync import Sync`

Change-Id: Ie10dd6832710939634c4f5c86b9ba5a9cd6fc92e

repo

@@ -88,7 +88,6 @@ REPO_MAIN = S_repo + '/main.py' # main script
 import optparse
 import os
 import re
-import readline
 import subprocess
 import sys
 import urllib2