refactor!: remove next/*.js exports

[hi-ogawa]

• partially revert fix(next): more compat exports #559

This one is added to help hi-ogawa/next-learn#1 (weird import by nextauth?) but it's fairly ugly (probably it confused IDE import completion too?) so let's forget about it and remove them.
Probably users can workaround it by doing alias manually.

I wonder what Next.js would end up eventually

• Add exports to package.json vercel/next.js#68455

---

Well, it looks like next-auth has fixed it already

• fix(next): fix Turbopack compat issue with next/*.js nextauthjs/next-auth#11551

[hi-ogawa]

• refactor!: remove next/*.js exports #647 👈
• main

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[hi-ogawa]

Tested in hi-ogawa/next-learn#1 and it looks just fine with:

{
  resolve: {
    alias: {
      // workaround next-auth's unusual import until
      // https://github.com/nextauthjs/next-auth/pull/11551
      'next/server.js': 'next/server',
      'next/headers.js': 'next/headers',
      'next/navigation.js': 'next/navigation',
    }
  }
}