Eta support for dhall-json

[jneira]

After #989 and #993 the use of the yaml package is isolated in the lib modules Dhall.Yaml and Dhall.YamlToDhall so it is easier to add support for compilers that cant use the yamlpackage like eta or ghcjs.
With this one we would add support for eta, a fork of ghc that compiles to jvm bytecode.
Main changes:

• Add conditional to cabal file and cpp conditions to the main modules for yaml to use a specific module for eta, Dhall.Yaml.Eta that replaces calls to the yamlpackage to ffi calls to the java lib jackson, one of the most popular ones in the java world.
• Add the java files that contains the ffi calls
• Mark buildable: False the subpackages that cant be built by eta for now

One effect of use the cabal file for cabal and etlas (the build tool for eta) is stack and cabal builds show those warnings:

Warning: Cabal file warning in D:\ws\eta\dhall\dhall-haskell\dhall-json\dhall-js
on.cabal@ 69:9:
         Unknown field: "maven-depends"

Warning: Cabal file warning in D:\ws\eta\dhall\dhall-haskell\dhall-json\dhall-js
on.cabal@ 74:9:
         Unknown field: "java-sources"

I've not found a way to avoid them other than use another file for etlas build (etlas.dhall). It would suppose duplicate most of the logic, though.

[Gabriella439]

@jneira: Thank you! Great work, as usual! 🙂