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
