Fix named-amd exports.

Ember does not export named-amd yet. We are shimming the `ember` module to have a single `default` export (this is exactly what EAK and ember-cli expect).

This means that using the following:

```javascript
import {get} from 'ember';
```

Which transpiles to:

```javascript
var get = __dependency1__.get;
```

Will not actually work since `__dependency1__` will only have a single property (`default`) and not a `get` property.

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This changes the imports to match reality. At some point, Ember will actually provide real ES6 (and possibly AMD) builds so I kept the variables that we were importing previously stashed at the top of each module (so that when we can do `import {get} from 'ember';` it is as simple as changing these same lines back).
Not sure how that would help/change anything. Ember CPM does not bundle Ember, it merely assumes that it exists already. This change just ensures that the output from our build is correct (instead of `var get = require('ember')['get'];` we have `var get = require('ember')['default'].get` which is correct).