[turbopack]: Fix HEAD requests (#50366)
I noticed while testing that we're getting a bunch of 500 errors after #50241 merged. Turns out that `fetchNextData` will [fetch `HEAD` requests](https://github.com/vercel/next.js/blob/cf9591cd/packages/next/src/shared/lib/router/router.ts#L619-L621) for background priority. The problem is that, somewhere, the Next router is draining the body from HEAD responses, leading us to trying to `JSON.parse` an empty string.
This changes the way we return results to the Turbopack router. Instead of `JSON.stringify`ing the result into the body (which will be drained by something), we directly return the result. And it saves us a `stringify` -> `parse` -> `stringify` round trip, so that's nice.
I took the chance to clean up some of our boilerplate code, too.
