tsickle output preserves shorthand imports

[alexeagle]

commonJS allows

folder/
  index.ts

to be imported with import {} from 'folder';
TypeScript allows the same convention, and in its --module=es6 output it preserves that import location.

However, this is unspecified and not supported by all bundlers, in particular closure compiler can't accept this ES6 output.

Tsickle is a good place for us to rewrite such imports to come from the canonical location.

[alexeagle]

Fixed by that PR.