External module name clashes

[Rich-Harris]

If you have a situation like this...

// main.js
import x from "moment";
import a from './a';

// a.js
var x = 'wut';
export default x;

...the x name isn't deconflicted between the reference that's internal to a.js, and the external module. Fix inbound

[Rich-Harris]

Fixed in 0.6.15