next dev --turbo does not respect pageExtensions for custom App/Document

[miselin]

Link to the code that reproduces this issue

https://github.com/miselin/next-turbo-pageextensions-repro

To Reproduce

1. npm run dev
2. Visit http://localhost:3000
3. "hi from _app" and "hi from _document" should be visible (DOM IDs app-ran-repro and document-ran-repro)
4. Terminate the dev server
5. npm run dev:turbo
6. Visit http://localhost:3000
7. The "hi from ..." tags are no longer visible in the rendered HTML. A hydration error is also shown from the MyApp component.

Current vs. Expected behavior

Currently, running with --turbo requires _app.tsx and _document.tsx regardless of the value of pageExtensions.

I expect these to also respect pageExtensions as they do when --turbo is not provided.

Verify canary release

• I verified that the issue exists in the latest Next.js canary release

Provide environment information

Operating System:
  Platform: darwin
  Arch: arm64
  Version: Darwin Kernel Version 22.6.0: Thu Nov  2 07:43:57 PDT 2023; root:xnu-8796.141.3.701.17~6/RELEASE_ARM64_T6000
Binaries:
  Node: 20.10.0
  npm: 10.2.3
  Yarn: 1.22.19
  pnpm: 8.11.0
Relevant Packages:
  next: 14.0.4-canary.39
  eslint-config-next: N/A
  react: 18.2.0
  react-dom: 18.2.0
  typescript: 5.1.3
Next.js Config:
  output: N/A

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

Turbopack (--turbo)

Additional context

No response

_PACK-2085

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