default to index.css instead of styles[0]

@MatthewMueller this is all it would take. 5 tests don't pass with this though so it will probably expand some. but, this solves the problem of being able to support both legacy component and the new `@import` style, so you can port stuff from component to duo without breaking anything in the process.
this looks like a bug. `'index.'` is always truthy