Move extensions.LabelSelector to unversioned.

Move type LabelSelector and type LabelSelectorRequirement from pkg/apis/extensions
This avoids an import loop when Job (and later DaemonSet, Deployment, ReplicaSet)
are moved out of extensions to new api groups.

Also Move LabelSelectorAsSelector utility from pkg/apis/extensions/ to pkg/api/unversioned/
Also its test.
Also LabelSelectorOp* constants.
Also the pkg/apis/extensions/validation functions ValidateLabelSelectorRequirement and
ValidateLabelSelector move to pkg/api/unversioned

The related type in pkg/apis/extensions/v1beta1/ is staying there.  I might move
it in another PR if neccessary.

pkg/controller/job/controller_test.go

@@ -45,7 +45,7 @@ func newJob(parallelism, completions int) *extensions.Job {
 			Namespace: api.NamespaceDefault,
 		},
 		Spec: extensions.JobSpec{
-			Selector: &extensions.LabelSelector{
+			Selector: &unversioned.LabelSelector{
 				MatchLabels: map[string]string{"foo": "bar"},
 			},
 			Template: api.PodTemplateSpec{
@@ -496,7 +496,7 @@ func TestJobPodLookup(t *testing.T) {
 			job: &extensions.Job{
 				ObjectMeta: api.ObjectMeta{Name: "foo"},
 				Spec: extensions.JobSpec{
-					Selector: &extensions.LabelSelector{
+					Selector: &unversioned.LabelSelector{
 						MatchLabels: map[string]string{"foo": "bar"},
 					},
 				},
@@ -515,11 +515,11 @@ func TestJobPodLookup(t *testing.T) {
 			job: &extensions.Job{
 				ObjectMeta: api.ObjectMeta{Name: "bar", Namespace: "ns"},
 				Spec: extensions.JobSpec{
-					Selector: &extensions.LabelSelector{
-						MatchExpressions: []extensions.LabelSelectorRequirement{
+					Selector: &unversioned.LabelSelector{
+						MatchExpressions: []unversioned.LabelSelectorRequirement{
 							{
 								Key:      "foo",
-								Operator: extensions.LabelSelectorOpIn,
+								Operator: unversioned.LabelSelectorOpIn,
 								Values:   []string{"bar"},
 							},
 						},