Move supervisor to it's own package

It allows to keep main namespace cleaner

Signed-off-by: Alexander Morozov <lk4d4@docker.com>

errors.go

@@ -1,24 +0,0 @@
-package containerd
-
-import "errors"
-
-var (
-	// External errors
-	ErrEventChanNil           = errors.New("containerd: event channel is nil")
-	ErrBundleNotFound         = errors.New("containerd: bundle not found")
-	ErrContainerNotFound      = errors.New("containerd: container not found")
-	ErrContainerExists        = errors.New("containerd: container already exists")
-	ErrProcessNotFound        = errors.New("containerd: processs not found for container")
-	ErrUnknownContainerStatus = errors.New("containerd: unknown container status ")
-	ErrUnknownEvent           = errors.New("containerd: unknown event type")
-
-	// Internal errors
-	errShutdown          = errors.New("containerd: supervisor is shutdown")
-	errRootNotAbs        = errors.New("containerd: rootfs path is not an absolute path")
-	errNoContainerForPid = errors.New("containerd: pid not registered for any container")
-	// internal error where the handler will defer to another for the final response
-	//
-	// TODO: we could probably do a typed error with another error channel for this to make it
-	// less like magic
-	errDeferedResponse = errors.New("containerd: defered response")
-)