cleanup(turbopack): Added documentation comments and small optimizations to CSS import validations (#80901)
## Refactor CSS Import Validation Logic in Next.js
### What?
Refactored the CSS import validation logic to improve readability and
maintainability by reorganizing the code flow and adding comprehensive
comments.
### Why?
The previous implementation had a confusing flow that made it difficult
to understand the validation rules for global CSS imports. This refactor
makes the logic clearer by:
1. Checking if the imported module is a global CSS module first
2. Validating imports from node_modules
3. Handling root nodes with no parents
4. Allowing imports from CSS modules
5. Finally checking if the parent is the app module
### How?
- Reordered the validation checks in `validate_pages_css_imports` to
follow a more optimized flow
- Added detailed comments explaining each validation rule and its
purpose
- Added documentation for the `validate_pages_css_imports` function
- Improved comments in the `pages.rs` file to explain why we recreate
the app_module
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Co-authored-by: Niklas Mischkulnig <4586894+mischnic@users.noreply.github.com>