WIP: implement packagesFrom

[zimbatm]

fixes #5

[paulovieira]

This new packagesFrom attribute would be useful. I hope this PR can be merged sometime in the future.

[srid]

This may be a tangential question, but how can I integrate numtide/devshell when already using a Haskell devshell that comes from developPackage? cf.

https://github.com/srid/haskell-template/blob/7f9fe6229507a68af21cb6201299099d83527346/flake.nix#L42-L43

https://github.com/srid/haskell-template/blob/7f9fe6229507a68af21cb6201299099d83527346/flake.nix#L79

I suppose I can merge them but that feels hacky.

[kenranunderscore]

Same question as @srid here: I'm using haskellPackages.shellFor at work and would like to use devshell in some way, but I'm not sure how to get the best of both worlds.