Always attempt to lookup index imports from current directory before existing paths

This patch makes sure import lookups always attempt to use a matching index module in the current directory before falling back to existing import paths.

Consider the following:

* components/index.styl (`@import "event"`)
* components/event.styl (`.event.listing { background: red }`)
* pages/index.styl (`@import "event"`)
* pages/event/index.styl (`.event { text-transform: uppercase }`)

```
@import "components"
@import "pages"
```

Without this patch, `pages/event/index.styl` would never get included in the resulting CSS since it would use the contents of `components/event.styl`.