Move unshared version code from helpers to version-cmd.py

While this is relatively "harmless", the import * directive brings in a
whole lot of unused code into the version subcommand.

At the same time, the three version_* functions are only used in the
version subcommand, so their place inside helpers.py is unjustified. The
same goes for the _version module.

Signed-off-by: Gabriel Filion <gabster@lelutin.ca>
[rlb@defaultvalue.org: adjust commit message]
Signed-off-by: Rob Browning <rlb@defaultvalue.org>