Change linux runtime(w shim) to builtin

Because of the plugin findings and having the default runtime builtin
this makes it much better for development and testing.

Signed-off-by: Michael Crosby <crosbymichael@gmail.com>

linux/container.go

@@ -1,4 +1,4 @@
-package main
+package linux
 
 import (
 	"github.com/docker/containerd"