Fix exports

[Rich-Harris]

It turns out that these are not equivalent:

declare module 'x' {
  type a = {...};
  export type b = {...};
}

declare module 'x' {
  type a = {...};
  type b = {...};

  export { b };
}

In the first case, everything is exported. There is no universe in which this makes sense. Happily, we can use the second form to get the correct behaviour.