Import names can be shadowed

[Rich-Harris]

Follow-up from #15 - if you have a named export, Esperanto will avoid naming collisions at the top level:

// this...
import { a } from 'foo';

a();
var foo = 'bar';

// ...becomes this
define(['foo'], function (_foo) {

  'use strict';

  _foo.a();
  var foo = 'bar';

});

But it's possible to shadow an import name:

// this...
import { a } from 'foo';

a();
(function () {
  var foo = 'bar';
  a();
}())

// ...becomes this, which won't work
define(['foo'], function (foo) {

  'use strict';

  foo.a();
  (function () {
    var foo = 'bar';
    foo.a();
  }())

});