Better support for source dependencies between cross-projects

[paulp]

Here's what I came up with:

def zioUri      = uri("git://github.com/scalaz/scalaz-zio.git#7f27e9cbc3")
lazy val zioJVM = ProjectRef(zioUri, "coreJVM")
lazy val zioJS  = ProjectRef(zioUri, "coreJS")

lazy val base = crossProject(JSPlatform, JVMPlatform)
  .jvmConfigure(_ dependsOn zioJVM)
  .jsConfigure(_ dependsOn zioJS)
  ...

But with a bit of support here, I think it could look like

lazy val zio = CrossProjectRef(
  uri("git://github.com/scalaz/scalaz-zio.git#7f27e9cbc3"),
  Map(JSPlatform -> "coreJS", JVMPlatform -> "coreJVM")
)
lazy val base = crossProject(JSPlatform, JVMPlatform)
  .dependsOn(zio)
  ...