message: [now-next] Compute Rewrites & Redirects Earlier
  This PR strictly moves code to make the diff for an upcoming PR cleaner.
diff: @@ -334,6 +334,27 @@ export const build = async ({
  env.NODE_OPTIONS = `--max_old_space_size=${memoryToConsume}`;
  await runPackageJsonScript(entryPath, shouldRunScript, { ...spawnOpts, env });
  + const routesManifest = await getRoutesManifest(entryPath, realNextVersion);
  + const rewrites: Route[] = [];
  + const redirects: Route[] = [];
  +
  + if (routesManifest) {
  + switch (routesManifest.version) {
  + case 1: {
  + redirects.push(...convertRedirects(routesManifest.redirects));
  + rewrites.push(...convertRewrites(routesManifest.rewrites));
  + break;
  + }
  + default: {
  + // update MIN_ROUTES_MANIFEST_VERSION in ./utils.ts
  + throw new Error(
  + 'This version of `@now/next` does not support the version of Next.js you are trying to deploy.\n' +
  + 'Please upgrade your `@now/next` builder and try again. Contact support if this continues to happen.'
  + );
  + }
  + }
  + }
  +
  if (isLegacy) {
  debug('Running npm install --production...');
  await runNpmInstall(
  @@ -819,8 +840,6 @@ export const build = async ({
  let dynamicPrefix = path.join('/', entryDirectory);
  dynamicPrefix = dynamicPrefix === '/' ? '' : dynamicPrefix;
  - const routesManifest = await getRoutesManifest(entryPath, realNextVersion);
  -
  const dynamicRoutes = await getDynamicRoutes(
  entryPath,
  entryDirectory,
  @@ -834,26 +853,6 @@ export const build = async ({
  })
  );
  - const rewrites: Route[] = [];
  - const redirects: Route[] = [];
  -
  - if (routesManifest) {
  - switch (routesManifest.version) {
  - case 1: {
  - redirects.push(...convertRedirects(routesManifest.redirects));
  - rewrites.push(...convertRewrites(routesManifest.rewrites));
  - break;
  - }
  - default: {
  - // update MIN_ROUTES_MANIFEST_VERSION in ./utils.ts
  - throw new Error(
  - 'This version of `@now/next` does not support the version of Next.js you are trying to deploy.\n' +
  - 'Please upgrade your `@now/next` builder and try again. Contact support if this continues to happen.'
  - );
  - }
  - }
  - }
  -
  return {
  output: {
  ...publicDirectoryFiles,
  