Turbopack: Tree shaking fixes and code organization (#89295)
## Summary
A series of bug fixes, refactors, and code organization improvements to Turbopack's tree shaking and side effect optimization:
- **Fix `ModulePart` handling for `FreeVarReference::EcmaScriptModule`** — pass `ModulePart` correctly so free variable references resolve to the right module part
- **Extract `EcmascriptModuleRenameModule`** — split the rename/reexport logic (`RenamedExport`, `RenamedNamespace`) out of `EcmascriptModuleFacadeModule` into a dedicated `rename` module, shared between side effects optimization and tree shaking
- **Simplify `EcmascriptModuleFacadeModule`** — now only handles the `Facade` case (locals + reexports), no longer overloaded with rename variants
- **Reorganize tree shaking code** — moved `asset.rs` → `part/module.rs`, `chunk_item.rs` → `part/chunk_item.rs` + `side_effects/chunk_item.rs`, `side_effect_module.rs` → `side_effects/module.rs` for clearer separation between part modules and side effect modules
- **Use `EsmExport` in `FindExportFromReexportsResult`** — reduces code duplication by reusing the existing `EsmExport` enum
- **Minor simplifications** — code deduplication, reordered checks, and general cleanup
