[turbopack] Add FileSystemPath.has_extension and optimize module graph operations (#81205)
## Optimize file extension checks and module graph traversal
### What?
This PR introduces several optimizations:
1. Added a new `has_extension()` method to `FileSystemPath` for more efficient extension checking
2. Improved module graph traversal with a dedicated `has_entry_module()` method
3. Reordered code in `AppProject` to avoid unnecessary clones
4. Simplified CSS resource handling in client reference manifest generation
### Why?
These changes improve performance by:
- Replacing string comparisons with more efficient suffix matching for file extensions
- Providing a direct way to check if a module is an entry point without iterating all entries
- Avoiding unnecessary clones operations when possible