Fix output files warning by Turborepo (#74811)
The new version of Turborepo (rightfully) complains that two of our
packages don't have output files specified when running `pnpm build`:
```
WARNING no output files found for task @next/codemod#build. Please check your `outputs` key in `turbo.json`
WARNING no output files found for task @vercel/turbopack-next#build. Please check your `outputs` key in `turbo.json`
```
The package `@next/codemod` emits its `.js`, `.js.map`, and `.d.ts`
files into the same folder as the respective source files instead of
`dist` (as specified in the root `turbo.json`). And
`@vercel/turbopack-next` does not emit any output files. Both facts are
now reflected in the two added package-specific `turbo.json` files.
