etlas project --json

[rahulmutt]

For interop with other build tools, it would be great if etlas can spit out the info in a .cabal file in a json format.

{"library" : {"hs-source-dirs":...}, "executable":{"name":"..", ...}

etlas project [target] should be thought of as a command that dumps the contents of the target's cabal file. If no target is specified, the current project's cabal file is used.

[jneira]

Talking with @rahulmutt we agreed in it would be useful that etlas project could generate the info for differents config formats: cabal, dhall or gradle (maybe dhall could replace json cause it is easily conveted to it)
Related with typelead/eta#720