'artifact checkout' has slightly different behaviour from 'checkout',
that is, either '--directory' or '--tar' are now required options.
This is a step towards allowing checkout to take multiple args.

This commit marks 'bst checkout' as a 'hidden' command. If used,
the user will be prompted to use the new 'bst artifact checkout'
command.

All tests which used 'bst checkout' have been modified to use
the new artifact sub-command.

This partially solves #822.