Enable code splitting for SSR build

[remorses]

This is a proposal for the next Remix breaking version.

Currently the server gets bundled in a single file, this can be slow to evaluate, slowing down startup.

It also makes impossible to enable sourcemaps on the server if your server code is too large, I always get OOM errors if I enable sourcemaps on ssr build.

My proposal is to put each route as an entrypoint in the SSR build, then change the remix-serve package to import the needed routes only instead of the whole server bundle, this will speed up startup time.

before

createRequestHandler({ build: await import(url.pathToFileURL(buildPath) })

after

let routesFilePaths = await getRouteFilesForPath(request.url)
createRequestHandler({ routes: await Promise.all(routesFilePaths.map(x => import(x)))  })