enforce the use of goimports (#248)

* enforce the use of goimports

Instead of enforcing `gofmt`, enforce `goimports`, which also asserts
a separate section for non-builtin packages.

* run `goimports` everywhere

* exclude generated .pb.go files

handshake_ix.go

@@ -1,11 +1,10 @@
 package nebula
 
 import (
+	"bytes"
 	"sync/atomic"
 	"time"
 
-	"bytes"
-
 	"github.com/flynn/noise"
 	"github.com/golang/protobuf/proto"
 )