Prevent merge conflicts with crate-hashes directory

[kolloch]

Currently, we cache hashes of prefetched packages (= crates) in crate-hashes.json. This is basically a simple list of package name / hash which can lead to many merge conflicts if dependencies keep changing.

If we saved the hashes in individual files in a crate-hashes directory, we would avoid this problem. crate2nix could search for existing hashes in all files and then write the still needed ones into a new file. It can then delete all the old files. This is easily mergeable. You might end up temporarily with multiple files but it will always converge to few or one file.

Currently, we also do NOT save any hash sums that we obtained from the lock file. For people who are not checking in lock files, this might be contraproductive. There is no real downside to saving all hashes.