grim/convey

Remove the convey/extend task as it's normal usage is already covered by plan/stage environment variables. It's corner case, expanding variables, is rather exotic and not something that comes up often.
tasks:
import-bad:
type: docker/import
files: /etc/ssl:/ssl
import:
type: docker/import
files: breakout.yml
export-bad:
type: docker/export
files: breakout.yml:/
export:
type: docker/export
files: breakout.yml:qux/quux/free.yml
plans:
default:
stages:
- tasks:
- import
- export
import-bad:
stages:
- tasks:
- import-bad
export-bad:
stages:
- tasks:
- import
- export-bad