perf(turbopack): improve app structure performance (#67397)
### What?
Improves the performance of loader tree creation.
The main change is `directory_tree_to_loader_tree` now being completely
synchronous (ignoring the setup).
This improves performance a lot (at least for the initial build), as
it's a recursive function, which was cached at every level before.
It also makes querying the loader tree easier, as not everything needs
to be an async turbo tasks function now.
#### `get_entrypoints` Performance
| | Initial | Update |
| --------- | ---------- | ------- |
| `canary` | `1300ms` | `20ms` |
| PR | `260ms` | `50ms` |
