This commit also ensures that if we try to use the 'old' commands,
BuildStream will fail and instruct the user to use the new command.

This implements a superset of the features of `bst checkout`,
but uses a --directory option rather than a location argument,
and --tar takes a location instead of being a flag.

If multiple element targets or any artifact refs are passed
then it does not support creating a tarball, running integration commands
or including dependencies of the targets.

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.