Add conflict driven priority reordering when backtracking

[notatallshaw]

If candidate packages are regular involved in a conflict they are promoted to being resolved first.

This makes significant performance improvements in many scenarios, e.g.

# On Python 3.12: ResolutionTooDeep -> 586 resolution rounds
pip install --dry-run --uploaded-prior-to "2025-11-17" 'trustllm'

# On Python 3.10: Build Failure -> Success
pip install --dry-run --uploaded-prior-to "2024-10-23" 'numpy==1.21.5' 'spacy<4.0.0,>=3.0.0' 'mlflow<3.0.0,>=2.13.0'

# On Python 3.10: 1257 resolution rounds -> 473 resolution  rounds
pip install --dry-run --uploaded-prior-to "2023-12-10" 'kedro[test]==0.18.13'

I could not find any examples where this made performance worse.

[notatallshaw]

FYI @frostming I assume this exact optimization can be applied to PDM. Also I plan to bring some bigger optimizations soon, probably to resolvelib.