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.go

@@ -15,10 +15,10 @@
 package main
 
 import (
-	"github.com/containernetworking/cni/pkg/ns"
 	"github.com/containernetworking/cni/pkg/skel"
 	"github.com/containernetworking/cni/pkg/types/current"
 	"github.com/containernetworking/cni/pkg/version"
+	"github.com/containernetworking/plugins/pkg/ns"
 	"github.com/vishvananda/netlink"
 )