Fix backjumping by checking if broken_state.mapping is empty

[notatallshaw]

Fixes #134

Still need to provide a test, going to see if I can write a small provider that can process the JSON in #134 (comment) and follows pip's preference logic.

But feedback would be welcome.

[notatallshaw]

Upon writing a test case I realized it wasn't sufficient to check if name is in incompatible dependencies, but the simpler check of checking if broken_state.mapping is empty is sufficient.

[notatallshaw]

Actually, while this solves the given test case, I found it still fails for "kedro[test]==0.18.13".

I am going to leave this PR as ready to review, because I still beleive it's correct to exit from backtracking when broken_state.mapping is empty.

But I am going to try and create another minimal example from "kedro[test]==0.18.13".

[notatallshaw]

I found it still fails for "kedro[test]==0.18.13"

After a bit of investigation I found that this is probably an edge case related to extras. I am investigating further, but it could be an issue on pip side. I plan to make a separate issue once I have found a simple reproducer. I would like this PR to be treated as solving the original example in #134