Deconflict shared helpers with user imports

[Rich-Harris]

Fixes one aspect of #222, wherein this...

<script>
  import { get } from 'ajax';
  export default {...};
</script>

...would create a conflict between the user's import and Svelte's built-in import in the case of shared helpers. Credit to @Swatinem who was right (as usual 😀 ) to point out that a method for creating helper aliases on-demand was the best way to use shared helpers.

[Swatinem]

Yes, looks nice! 👍