[internal] use es6 exports

prevent rollup warning

lib/builder/index.js

@@ -1,11 +1,6 @@
 import Builder from './builder'
 import Generator from './generator'
 
-export default {
-  Builder,
-  Generator
-}
-
 export {
   Builder,
   Generator

lib/core/index.js

@@ -3,14 +3,6 @@ import Module from './module'
 import Nuxt from './nuxt'
 import Renderer from './renderer'
 
-export default {
-  Nuxt,
-  Module,
-  Renderer,
-  Options,
-  Utils
-}
-
 export {
   Nuxt,
   Module,

lib/index.js

@@ -1,4 +1,4 @@
-import core from './core'
+import * as core from './core'
-import builder from './builder'
+import * as builder from './builder'
 
 export default Object.assign({}, core, builder)