[wasm-split] Don't use ParallelFunctionAnalysis in scanModule (#9007)

This removes the use of `ParallelFunctionAnalysis` within `scanModule`
(in `computeUsedNames`), which scans `UsedNames` for each module.

I'm not 100% sure why but this improves running time at least for Dart
applications. I also previously tried to use `ParallelFunctionAnalysis`
in other functions but it resulted in slowdown so didn't do it. Maybe
cache locality works against the parallelism.

This reduces running time of acx_gallery (Jul 2026) by 7.8% (30.6s ->
28.2s) essentials by 4.2% (225.1s -> 215.6s).