Bundler creates unused chunk

## Description
When bundling with splitting enabled, a chunk is created for every dynamic import even if it's not used.

## Expected behavior
Given the output `entry.js` is empty, there should be no chunks generated as they're not being referenced.
I added a `sideEffects: false` as well to indicate that `file-b.js` has no side effect.

When disabling splitting, it works expectedly (chunk is removed from being inlined in entry).

## Reproduction
https://esbuild.github.io/try/#YgAwLjE4LjE0AHsKICBidW5kbGU6IHRydWUsCiAgc3BsaXR0aW5nOiB0cnVlLAogIGZvcm1hdDogJ2VzbScsCiAgb3V0ZGlyOiAnLicsCn0AZQBlbnRyeS5qcwBleHBvcnQge30gZnJvbSAnLi9maWxlLWEuanMnAABmaWxlLWEuanMAZXhwb3J0IGNvbnN0IGEgPSAoKSA9PiBpbXBvcnQoJy4vZmlsZS1iLmpzJyk7AABmaWxlLWIuanMAZXhwb3J0IGNvbnN0IGIgPSAxOwAAcGFja2FnZS5qc29uAHsgInNpZGVFZmZlY3RzIjogZmFsc2UgfQ

Start by reproducing the linked case in the esbuild playground, using entry.js, file-a.js, file-b.js, and package.json, and inspect the generated entry and split chunks. Trace how dynamic imports and sideEffects: false affect splitting and tree shaking; done means the unused chunk is not generated while referenced chunks still work.

Written by the indexing model from the issue text.