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/ipam/dhcp/lease.go

@@ -26,8 +26,8 @@ import (
 	"github.com/d2g/dhcp4client"
 	"github.com/vishvananda/netlink"
 
-	"github.com/containernetworking/cni/pkg/ns"
 	"github.com/containernetworking/cni/pkg/types"
+	"github.com/containernetworking/plugins/pkg/ns"
 )
 
 // RFC 2131 suggests using exponential backoff, starting with 4sec