Fix context package imports

Conflicting with definition of plugin.Differ

Signed-off-by: Tonis Tiigi <tonistiigi@gmail.com>

rootfs/apply.go

@@ -1,7 +1,6 @@
 package rootfs
 
 import (
-	"context"
 	"fmt"
 
 	"github.com/containerd/containerd/log"
@@ -11,6 +10,7 @@ import (
 	"github.com/opencontainers/image-spec/identity"
 	ocispec "github.com/opencontainers/image-spec/specs-go/v1"
 	"github.com/pkg/errors"
+	"golang.org/x/net/context"
 )
 
 type Applier interface {

rootfs/diff.go

@@ -1,13 +1,13 @@
 package rootfs
 
 import (
-	"context"
 	"fmt"
 
 	"github.com/containerd/containerd/content"
 	"github.com/containerd/containerd/mount"
 	"github.com/containerd/containerd/snapshot"
 	ocispec "github.com/opencontainers/image-spec/specs-go/v1"
+	"golang.org/x/net/context"
 )
 
 type MountDiffer interface {

services/diff/client.go

@@ -1,14 +1,13 @@
 package diff
 
 import (
-	"context"
-
 	diffapi "github.com/containerd/containerd/api/services/diff"
 	"github.com/containerd/containerd/api/types/descriptor"
 	mounttypes "github.com/containerd/containerd/api/types/mount"
 	"github.com/containerd/containerd/mount"
 	"github.com/containerd/containerd/rootfs"
 	ocispec "github.com/opencontainers/image-spec/specs-go/v1"
+	"golang.org/x/net/context"
 )
 
 type DiffService interface {