Link to the code that reproduces this issue

https://github.com/devjiwonchoi/repro-next-turbo-config-pkg

To Reproduce

1. Install the dependencies
2. Run pnpm dev
3. The TEXT CENTER text is not centered, postcss config from package.json is not picked up, tailwindcss is not working
4. Run pnpm next dev
5. The TEXT CENTER text is centered, postcss config from package.json is picked up, tailwindcss is working

Current vs. Expected behavior

Turbopack is not picking up the postcss config inside the package.json, resulting the tailwind to not work.
The webpack-based build (next dev) does pick it up and works as expected.

Provide environment information

Operating System:
  Platform: darwin
  Arch: arm64
  Version: Darwin Kernel Version 23.5.0: Wed May  1 20:13:18 PDT 2024; root:xnu-10063.121.3~5/RELEASE_ARM64_T6030
  Available memory (MB): 36864
  Available CPU cores: 12
Binaries:
  Node: 18.17.0
  npm: 9.6.7
  Yarn: 1.22.19
  pnpm: 8.15.4
Relevant Packages:
  next: 15.0.0-canary.45 // Latest available version is detected (15.0.0-canary.45).
  eslint-config-next: N/A
  react: 19.0.0-rc-6f23540c7d-20240528
  react-dom: 19.0.0-rc-6f23540c7d-20240528
  typescript: 5.3.3
Next.js Config:
  output: N/A

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

Turbopack

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

next dev (local)

Additional context

No response