Find all references does not work for default export of ambient module

```
// declarations.d.ts
declare module "x" {
    const df: number;
    export default df;
}

// index.ts
import df from "x";
```

Goto-declaration works for `df` in `index.ts`, but find-all-references for `df` in `declarations.d.ts` only finds the two uses in that file.
