Move supervisor to it's own package

It allows to keep main namespace cleaner

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

supervisor/supervisor_linux.go

@@ -0,0 +1,12 @@
+// +build libcontainer
+
+package supervisor
+
+import (
+	"github.com/docker/containerd/linux"
+	"github.com/docker/containerd/runtime"
+)
+
+func newRuntime(stateDir string) (runtime.Runtime, error) {
+	return linux.NewRuntime(stateDir)
+}