plugins: correctly output build version, cosmetic cleanups

Now that libcni has the ability to print a version message, plumb it
through correctly.

While we're at it,
- fix import paths
- run gofmt
- add some more comments to sample
- add container runtime swappability for release

plugins/main/loopback/loopback.go

@@ -15,11 +15,14 @@
 package main
 
 import (
+	"github.com/vishvananda/netlink"
+
 	"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"
+	bv "github.com/containernetworking/plugins/pkg/utils/buildversion"
 )
 
 func cmdAdd(args *skel.CmdArgs) error {
@@ -71,8 +74,7 @@ func cmdDel(args *skel.CmdArgs) error {
 }
 
 func main() {
-	// TODO: implement plugin version
-	skel.PluginMain(cmdAdd, cmdCheck, cmdDel, version.All, "TODO")
+	skel.PluginMain(cmdAdd, cmdCheck, cmdDel, version.All, bv.BuildString("loopback"))
 }
 
 func cmdCheck(args *skel.CmdArgs) error {