Turbopack: disable tree shaking for tracing (#85722)
We had tree shaking enabled for NFT.
That is incorrect however, as even unused imports (if you don't use any of the imported bindings, and the file is marked as `sideEffects: false`) are still needed at runtime, since Node.js will execute everything anyway.
- I made the `treeShakingMode: None` explicit now (though that was already the `Default::default` anyway)
- The real problem was that we still dropped side-effect free `ModulePart::Evaluation` references even when tree shaking was disabled.
Closes #85172
Fixes PACK-5756
