Cargo.lock not removed if git dependencies has been removed

[pbsds]

While making this pr i noticed nix-update update the vendored lock file, instead of removing it.

[Mic92]

Not sure we should "fix" this. We are not able to tell if it was changed to cargoHash and than deleting some random Cargo.lock might actually upset the user.

[Mic92]

We would need to evaluate before and see if there had been cargoLock attributes and only drop it in that case.

[pbsds]

Is there a reason to prefer cargoLock over cargoHash? Cache reuse?

[Artturin]

Is there a reason to prefer cargoLock over cargoHash? Cache reuse?

Only if the lock has git dependencies in which case outputHashes is required

[pbsds]

There is also the fact that with cargoLock each dep becomes a derivation, making for better cache reuse. But on the whole I'd say its speed and cache benefits are not worth vendoring lock files, and it's not like we make use of the multiple derivations in any other way.