move most of cni/pkg to plugins/pkg: delete code staying in cni/pkg

This moves the following packages to this repository:
* ip
* ipam
* ns
* testutils
* utils

plugins/main/loopback/loopback_test.go

@@ -20,7 +20,7 @@ import (
 	"os/exec"
 	"strings"
 
-	"github.com/containernetworking/cni/pkg/ns"
+	"github.com/containernetworking/plugins/pkg/ns"
 	. "github.com/onsi/ginkgo"
 	. "github.com/onsi/gomega"
 	"github.com/onsi/gomega/gbytes"