apply `flags` from `stack.yaml`

[cdepillabout]

stack.yaml has a top-level flags key to specify Cabal flags to pass to any package:

https://docs.haskellstack.org/en/stable/configure/yaml/project/#flags

It would be nice to for stacklock2nix to interpret this list of flags and apply them automatically to the packages in the package set it generates.

[Mr-Andersen]

Same goes for allow-newer/allow-newer-deps? It seems currently derivations need to be jailbroken on Nix side.