Type alias spec in oci package

This allows Go to build third party packages correctly without vendoring
issues what want to create their own SpecOpts.

Fixes #2289

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

container.go

@@ -28,9 +28,9 @@ import (
 	"github.com/containerd/containerd/cio"
 	"github.com/containerd/containerd/containers"
 	"github.com/containerd/containerd/errdefs"
+	"github.com/containerd/containerd/oci"
 	"github.com/containerd/typeurl"
 	prototypes "github.com/gogo/protobuf/types"
-	specs "github.com/opencontainers/runtime-spec/specs-go"
 	"github.com/pkg/errors"
 )
 
@@ -45,7 +45,7 @@ type Container interface {
 	// NewTask creates a new task based on the container metadata
 	NewTask(context.Context, cio.Creator, ...NewTaskOpts) (Task, error)
 	// Spec returns the OCI runtime specification
-	Spec(context.Context) (*specs.Spec, error)
+	Spec(context.Context) (*oci.Spec, error)
 	// Task returns the current task for the container
 	//
 	// If cio.Attach options are passed the client will reattach to the IO for the running
@@ -126,12 +126,12 @@ func (c *container) SetLabels(ctx context.Context, labels map[string]string) (ma
 }
 
 // Spec returns the current OCI specification for the container
-func (c *container) Spec(ctx context.Context) (*specs.Spec, error) {
+func (c *container) Spec(ctx context.Context) (*oci.Spec, error) {
 	r, err := c.get(ctx)
 	if err != nil {
 		return nil, err
 	}
-	var s specs.Spec
+	var s oci.Spec
 	if err := json.Unmarshal(r.Spec.Value, &s); err != nil {
 		return nil, err
 	}