Move runtime implementation types to pkg

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

runtime/runtime.go

@@ -0,0 +1,9 @@
+package runtime
+
+import "github.com/opencontainers/specs"
+
+// runtime handles containers, containers handle their own actions.
+type Runtime interface {
+	Create(id, bundlePath string, stdio *Stdio) (Container, error)
+	StartProcess(Container, specs.Process, *Stdio) (Process, error)
+}