Improve error message and collision detection logic for yarn tarball collisions

[slimreaper35]

This issue addresses feedback from #786, which aims to improve user experience and code quality in handling yarn classic tarball collisions. See the comments.

The current error message reports duplicate tarball filenames (e.g., example.tgz) but does not list the specific packages causing the collision. Users are forced to manually search yarn.lock to identify the problematic packages, which is not user-friendly.

The existing logic for detecting collisions could be refactored to use native set-based filtering or hashable models for faster deduplication.

[eskultety]

Just a heads-up that this will primarily need to address the fact that we currently approach things in an "agent 007" style - we first download all dependencies directly to the offline mirror, potentially overwriting an existing tarball due to a collision, and only then try to detect those collisions which is when we may not have the correct colliding packages names at hand anymore because we sometimes need to pull information about packages directly from the downloaded tarballs.