Turbopack: refactoring to prepare for name mangling (#89406)
## What
Refactors module splitting and re-export handling to prepare for the name mangling PR.
### Changes
1. **`EsmExports::reexport_including_default` helper** — New helper that creates re-export facades (default + star export) from a module reference. Replaces duplicated boilerplate in `NextDynamicEntryModule`, `NextServerComponentModule`, and `NextServerUtilityModule`.
2. **`get_locals_if_split` / `get_facade_if_split` on `EcmascriptModuleAsset`** — Encapsulates the split-check-then-wrap logic that was previously inlined in `turbopack/src/lib.rs`. Returns the locals or facade module when the module has re-exports that should be separated, otherwise returns self.
3. **Simplified `apply_module_type` in `turbopack/src/lib.rs`** — The tree-shaking `ReexportsOnly` path now uses the new helpers instead of manually checking `split_locals_and_reexports` and constructing facade/locals modules inline.
4. **Documentation improvements** — Added doc comments to `EsmExport` variants, `EsmExports` fields, and `split_locals_and_reexports`.
## Why
Extracting these helpers simplifies the upstream code and reduces the diff for the name mangling PR. The `reexport_including_default` helper is needed because the next stage will require additional work for virtual facade modules to function with mangled names. The `get_locals_if_split` / `get_facade_if_split` methods centralize split logic so that next.js module wrappers (which need unmangled export names) can use them directly.
