Move util.StringSet into its own package

A lot of packages use StringSet, but they don't use anything else from
the util package. Moving StringSet into another package will shrink
their dependency trees significantly.

pkg/expapi/validation/validation.go

@@ -25,6 +25,7 @@ import (
 	"k8s.io/kubernetes/pkg/labels"
 	"k8s.io/kubernetes/pkg/util"
 	errs "k8s.io/kubernetes/pkg/util/fielderrors"
+	"k8s.io/kubernetes/pkg/util/sets"
 )
 
 // ValidateHorizontalPodAutoscaler can be used to check whether the given autoscaler name is valid.
@@ -79,7 +80,7 @@ func ValidateThirdPartyResource(obj *expapi.ThirdPartyResource) errs.ValidationE
 	if len(obj.Name) == 0 {
 		allErrs = append(allErrs, errs.NewFieldInvalid("name", obj.Name, "name must be non-empty"))
 	}
-	versions := util.StringSet{}
+	versions := sets.String{}
 	for ix := range obj.Versions {
 		version := &obj.Versions[ix]
 		if len(version.Name) == 0 {