libcontainerd has a bunch of platform dependent code and huge interfaces that are a pain implement. To make the plugin manager a bit easier to work with, extract the plugin executor into an interface and move the containerd implementation to a separate package. Signed-off-by: Brian Goff <cpuguy83@gmail.com> Upstream-commit: c85e8622a4813d7b72d74517faa03ab5de4c4550 Component: engine