Add `find_shortest_cycle`

[Peter554]

This PR adds the find_shortest_cycle method to grimp, which I've been proposing in e.g. https://github.com/seddonym/grimp/issues/189.

This is a low-level, granular method for finding cycles in the import graph. We could potentially add some higher order methods for working with import cycles across the entire graph as @K4liber has been suggesting - I think we can potentially have both, e.g. similar to the import chain APIs find_shortest_chain, find_shortest_chains and find_illegal_dependencies_for_layers (I think grimp should offer both lower and higher level APIs - we can add higher level APIs, but we also shouldn't forget/miss the more basic APIs - for one thing not everyone is using grimp with import linter).

Still TODO:

• [ ] Align with @seddonym and @K4liber on whether we want this method added to grimp
• [ ] Check implementation for correctness. More (python) tests?
• [ ] Add benchmark
• [ ] Usage docs
• [ ] Update changelog

[codspeed-hq[bot]]

CodSpeed Performance Report

Merging #194 will not alter performance

_{Comparing Peter554:find-shortest-cycle (43b7c31) with master (8c1bb5d)}

Summary

✅ 17 untouched benchmarks

[Peter554]

I'm going to close this one as it's a bit stale and I'm not working on it.

[K4liber]

@Peter554 I am actually using your work while implementing AcyclicContract (still a draft, but soon it will be ready for a review: https://github.com/seddonym/import-linter/pull/250). It seems that it works as intended. Maybe one thing could be improved which is a deterministic output. Right now it seems to not be deterministic. I get different "shortest cycle" between runs for the exact same dependency graph.