Turbopack: don't cell CodeGen-s (#75513)
No need to cell them, they are module-specific anyway.
```
testing against 0293c96cf32
canary dd676ccbdd:
14,7 GB
TURBO_ENGINE_READ_ONLY=1 NEXT_TURBOPACK_TRACING= TURBOPACK=1 TURBOPACK_BUILD= 533.84s user 84.44s system 834% cpu 1:14.10 total
TURBO_ENGINE_READ_ONLY=1 NEXT_TURBOPACK_TRACING= TURBOPACK=1 TURBOPACK_BUILD= 526.68s user 86.15s system 834% cpu 1:13.48 total
plus fewer cells b5ce9afeb5:
14,3 GB
TURBO_ENGINE_READ_ONLY=1 NEXT_TURBOPACK_TRACING= TURBOPACK=1 TURBOPACK_BUILD= 517.74s user 85.77s system 831% cpu 1:12.54 total
TURBO_ENGINE_READ_ONLY=1 NEXT_TURBOPACK_TRACING= TURBOPACK=1 TURBOPACK_BUILD= 516.06s user 85.35s system 836% cpu 1:11.89 total
plus no self reads b3370a59ca:
13,8 GB
TURBO_ENGINE_READ_ONLY=1 NEXT_TURBOPACK_TRACING= TURBOPACK=1 TURBOPACK_BUILD= 516.65s user 84.11s system 834% cpu 1:12.01 total
TURBO_ENGINE_READ_ONLY=1 NEXT_TURBOPACK_TRACING= TURBOPACK=1 TURBOPACK_BUILD= 518.01s user 87.55s system 839% cpu 1:12.15 total
```
<details>
<summary>Old</summary>
```
canary 82026906a6:
15,3gb
492.59s user 103.43s system 736% cpu 1:20.90 total
495.16s user 96.50s system 739% cpu 1:20.01 total
code-gen bd58ad63a7:
14,95gb
489.30s user 102.66s system 745% cpu 1:19.36 total
501.91s user 105.99s system 743% cpu 1:21.80 total
```
</details>
