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

lighthouse_test.go

@@ -4,7 +4,7 @@ import (
 	"net"
 	"testing"
 
-	proto "github.com/golang/protobuf/proto"
+	"github.com/golang/protobuf/proto"
 	"github.com/stretchr/testify/assert"
 )