Return incompatibility from conflict & Allow backtracking to before a specific package

[konstin]

Background reading: astral-sh/uv#8157

I'll link the PR on the uv side with all the details once it's up, it contains a walkthrough.

This adds two features to pubgrub:

Return incompatibility from conflict: Whenever we either discard a version due to its dependencies or perform conflict resolution, we return the last conflict that led to discarding them. In uv, we use this to re-prioritize and backtrack when a package decision accumulated to many conflicts. Using the incompatibilities directly from pubgrub makes this efficient.

Allow backtracking to before a specific package: Once we identified a pair of conflicting packages, we want to switch their priorities. This requires backtracking to before the decision for the earlier of the two packages was made. On the uv side, we change the priorities to make sure we take a different path on the next attempt. We allow attempting to backtrack on packages that were not decided yet to avoid the caller from making the duplicative check: At the time when the see incompatibilities caused by them, they are not necessarily decided.

Best reviewed commit-by-commit.

[codspeed-hq]

CodSpeed Performance Report

Congrats! CodSpeed is installed 🎉

🆕 6 new benchmarks were detected.

You will start to see performance impacts in the reports once the benchmarks are run from your default branch.

Detected benchmarks

• backtracking_disjoint_versions (2.3 s)
• backtracking_ranges (2 s)
• backtracking_singletons (4.3 s)
• large_case_u16_NumberVersion.ron (21.4 ms)
• sudoku-easy (3.8 ms)
• sudoku-hard (3.9 ms)