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

subcmds/cherry_pick.py

@@ -13,7 +13,8 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-import sys, re
+import re
+import sys
 from command import Command
 from git_command import GitCommand
 

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

subcmds/version.py

@@ -16,7 +16,7 @@
 import sys
 from command import Command, MirrorSafeCommand
 from git_command import git
-from project import HEAD
+from git_refs import HEAD
 
 class Version(Command, MirrorSafeCommand):
   wrapper_version = None