Parse version numbers for better diffs

[9999years]

Right now, if a derivation version changes, nix-diff just says that the sets of derivations changed. If nix-diff were to parse derivation names into package names and version numbers, it would be able to determine that the version of a package changed and provide a better diff.

[SuperSandro2000]

Maybe we could re-use the logic that diff-closures behind the back uses for that.