This is a first step towards loading more stuff simultaneously rather
than sequentially.
The functional part of this is pretty small: call `calculateFetchPaths` and
then `Promise.all(fetchPaths.map(loadFileFromURL));`. I also transposed the
return type of `calculateFetchPaths` since it's more convenient to consume
this way.
I redid the logic in `calculateFetchPaths` a bit. I renamed `src/plugins/fetch.ts`
to `calculateFetchPaths.ts` since the file performs no fetching. I also
renamed `loadFromFile` to `loadFileFromURL`.
