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
diff --git a/subcmds/smartsync.py b/subcmds/smartsync.py
index 1edbd35..e164859 100644
--- a/subcmds/smartsync.py
+++ b/subcmds/smartsync.py
@@ -13,7 +13,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-from sync import Sync
+from subcmds.sync import Sync
 
 class Smartsync(Sync):
   common = True