Add a `checker` package that adds some utility Checker implementation,
the first one being `checker.Contains`, as well as brining all go-check
provided Checker implementations in scope as a commodity.
Signed-off-by: Arnaud Porterie <arnaud.porterie@docker.com>
Upstream-commit: c87cbd3862336fb01addff5d753ff7c8e0d50879
Component: engine
