allow importing "server-only" modules in next.config.js

[stefanprobst]

Link to the code that reproduces this issue

https://github.com/stefanprobst/issue-next-config-server-import

To Reproduce

1. clone https://github.com/stefanprobst/issue-next-config-server-import and run pnpm install
2. pnpm run dev
3. see error message:

 ⨯ Failed to load next.config.js, see more info here https://nextjs.org/docs/messages/next-config-error

> Build error occurred
Error: This module cannot be imported from a Client Component module. It should only be used from a Server Component.

Current vs. Expected behavior

i am trying to import a module which has import "server-only"; into next.config.js and get the following error:

 ⨯ Failed to load next.config.js, see more info here https://nextjs.org/docs/messages/next-config-error

> Build error occurred
Error: This module cannot be imported from a Client Component module. It should only be used from a Server Component.

i had assumed that next.config.js would not be read client side.

i also checked whether next.config.js is actually read client-side by adding the following code in next.config.js (and everything still worked, so i guess importing from server-only modules should be ok):

if (typeof document !== "undefined") {
  throw new Error("Reading next.config.js on the client.");
}

Provide environment information

Operating System:
  Platform: linux
  Arch: x64
  Version: #202403151937 SMP PREEMPT_DYNAMIC Fri Mar 15 19:52:22 UTC 2024
  Available memory (MB): 31401
  Available CPU cores: 16
Binaries:
  Node: 20.12.2
  npm: 10.5.0
  Yarn: 1.22.19
  pnpm: 9.0.6
Relevant Packages:
  next: 14.3.0-canary.57 // Latest available version is detected (14.3.0-canary.57).
  eslint-config-next: N/A
  react: 18.3.1
  react-dom: 18.3.1
  typescript: 5.4.5
Next.js Config:
  output: N/A

Which area(s) are affected? (Select all that apply)

Module Resolution

Which stage(s) are affected? (Select all that apply)

next dev (local)

Additional context

also see this discussion: #65605 (comment)