Cross compile support

[charleso]

Currently the plugin will generate the relevant dependencies for the select version of Scala.

We need to respect the cross-compile settings in SBT and generate multiple attributes for each module and make it explicit.

{ sbt ? import ../sbt.nix {}, deps ? import ../deps.nix { inherit sbt; } }:
let
  hello-bar = import ../bar { inherit sbt; };
in {
2_11 = sbt.mkDerivation {
  pname = "hello-foo";
  scalaVersion = sbt.scala_2_11;
  sources = [ ./src/main/scala ./src/main/scala-2.11 ]
  modules = [ hello-bar ];
};
2_10 = sbt.mkDerivation {
  pname = "hello-foo-2_10";
  sources = [ ./src/main/scala ./src/main/scala-2.10 ]
  scalaVersion = sbt.scala_2_10;
  modules = [ hello-bar ];
};
}

And the same again in the top-level aggregate project.

If there's a way to avoid some/all of that duplication that would be awesome. :)