Turbopack: optimize client manifest generation (#75248)
**View diff without whitespace changes!**
This brings the number of `AssetIdent::path` calls down from 5M to 100k
```
### canary (de984c2695b):
~16,60GB
582.76s user 109.72s system 815% cpu 1:24.95 total
583.30s user 108.45s system 805% cpu 1:25.86 total
### plus mischnic/client-manifest-optimization:
still ~16,60GB
547.70s user 105.24s system 811% cpu 1:20.48 total
550.16s user 108.14s system 782% cpu 1:24.10 total
```
