The way we currently track server CSS imports is to collect CSS files that each **module** depends on. This happens on the module graph level which is a global thing and cannot be tree-shaken properly (check the enabled test for more details).
In this PR we collect another information, of CSS files that each **entrypoint** depends on. This is the CSS list after tree-shaken on the entry level. By intersecting these CSS imports with the module-level CSS imports, we can get the final used CSS imports for each _layout_.
cc @hanneslund
## Bug
- [ ] Related issues linked using `fixes #number`
- [x] Integration tests added
- [ ] Errors have a helpful link attached, see `contributing.md`
## Feature
- [ ] Implements an existing feature request or RFC. Make sure the feature request has been accepted for implementation before opening a PR.
- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Documentation added
- [ ] Telemetry added. In case of a feature if it's used or not.
- [ ] Errors have a helpful link attached, see `contributing.md`
## Documentation / Examples
- [ ] Make sure the linting passes by running `pnpm lint`
- [ ] The "examples guidelines" are followed from [our contributing doc](https://github.com/vercel/next.js/blob/canary/contributing/examples/adding-examples.md)
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