Rename import path to `ember-fetch`

It's a best practice in general, for imports to come from their package name rather than some more "convenient" name, and will become more so [as Embroider lands](https://github.com/emberjs/rfcs/blob/embroider/text/0000-embroider-v2-package-format.md#renaming-rules):

> New addons *should not* use renaming rules because it's confusing when the imports people type don't align with their real dependencies.

The import style here should simply be:

```js
import fetch from 'ember-fetch';
```

This is not a blocker for use with Embroider, as there are solutions to support the existing flow, but is a nice improvement and will eliminate one thing that has to be done for this to use Embroider. It also has the nice upside of eliminating *all* shenanigans required to have this do the right thing for TypeScript integration!

This would be a breaking change, but should also be trivially codemod-able.