Allow backtracking to before a specific package

[konstin]

This allows discarding a previously made decision if it turned out to be a bad decision, even if all options with this decision have not yet been rejected.

We allow attempting to backtrack on packages that were not decided yet to avoid the caller from making the duplicative check.

astral-sh/uv#8157

[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 (21.3 ms)
• sudoku-easy (3.8 ms)
• sudoku-hard (3.9 ms)