Hello,
Current code:

1. Creates new dependency_cache dictionary inside solve_dependencies function if passed dependency_cache parameter is None or empty.
2. As a result of (1) dependency_cache.update(solved_result.dependency_cache) is required at every iteration of dependencies solving, because solved_result.dependency_cache returned by recursion can be either same as dependency_cache (if at least one dependency was already stored in cache) or new dict (on first iterations of recursion where empty cache is passed to recursive function)

This behaviour adds a bit complexity to the code and spends (albeit a little) more CPU and memory resources.

My proposal:

1. always create dependency cache on first iteration of recursive dependency solving and pass created cache instance to all subsequent calls. Do not exchange cache dict on first steps of recursion iterations if passed dict was empty.
2. Now dependency_cache.update(solved_result.dependency_cache) is not required, because one dependency_cache is used over all dependency solving process.