Support Module Rewriting on Externs

It's been the norm for CommonJS for a long time and now with ES6 it will be even more prevalent. I think the ES6 and CommonJS module rewriting should be done on externs. This should pave the way to support non-global extern types.

Note: this would be a breaking change for the Node externs as they have export statements which would actually trigger the rewriting.

The final goal for this is to be able to have non-global typed externs and to be able to use those types in code:

``` js
var externalRequire = require;

/** @type {externs.zip//nodejs/http/http} http */
var http = externalRequire("http");
```