This is because in a nomal workflow, you would generate the .cabal every now and then from the package.yaml file. If you forget to do this generation and you have an old .cabal file in your tree (common because for users of hpack, .cabal files are usually in .gitignore), then until now nix would pick up the outdated cabal file and you would accidentally be running against outdated cabal contents. So in this commit we change the logic from "if *.cabal doens't exist, check if package.yaml exist, if so use hpack" to "if package.ymal exists, use hpack, otherwise use *.cabal"