Move npc/ipset package to net/ipset

It's going to be used by net/bridge.go.

npc/controller_test.go

@@ -6,7 +6,7 @@ import (
 
 	"github.com/pkg/errors"
 	"github.com/stretchr/testify/require"
-	"github.com/weaveworks/weave/npc/ipset"
+	"github.com/weaveworks/weave/net/ipset"
 	coreapi "k8s.io/api/core/v1"
 	networkingv1 "k8s.io/api/networking/v1"
 	metav1 "k8s.io/apimachinery/pkg/apis/meta/v1"