Resolve Python Dependency Versions

Problem: Python Dependency Version Resolution Given a set of Python package dependencies and version constraints (e.g., A depends on B>=1.2,<2.0), implement a dependency resolver that computes an installable set/plan and detects conflicts. The interview problem has 4 sub-questions that progressively add features. The implementation is code-heavy and focuses on correctness and engineering rather than optimization. Implement the required functions according to the provided input format and make the given tests pass. (Note: the original post did not include exact I/O format nor the 4 sub-questions; only the confirmed intent is captured here.)