[etlas] Migrate to hpack

[rahulmutt]

https://github.com/sol/hpack is a wonderful tool to convert yaml files into Cabal files. The syntax for Cabal files can be adhoc at times so having something structured and standardized like yaml seems like a great fit. It seems like a great idea for Etlas to make a yaml-like format the default.

[NickSeagull]

Another interesting idea would be if etlas handled the conversion to the cabal file internally.

I mean, no cabal file would be generated in the root of the project, or at least unless specified. This would lead to less confusions in my opinion. "Why do you have two project config files?"

[rahulmutt]

That was intended above - I was probably not clear. The idea is to eventually move to a package.yaml file as the default at the root of the directory. No need to even use a Cabal file internally - all you need is a user-facing representation of the build model and parsing a package.yaml is enough for that.

[jneira]

Maybe we can consider the use of dhall instead (directly or using dhall-to-cabal), as commented in #704.

[NickSeagull]

I like dhall, but sometimes it looks really verbose. We should abstract stuff from it.

[jneira]

Well i think that finally dhall would be the chosen format (see typelead/etlas#66) so it seems we can close this one.