Semi-port of "mvn dependency:list -Dsort=true -DoutputFile=deps.mvn.txt" to sbt

dependencyList.sbt

val dependencyList = taskKey[File]("")

val dependencyListDef = Def.task {
  val f = baseDirectory.value / "deps.sbt.txt"
  streams.value.log.info(s"Writing to $f")
  IO.write(f, "\n")
  IO.append(f, "The following files have been resolved:\n")
  val cp = (managedClasspath in Test).value
  val lines = cp map { att =>
    val md = att.metadata
    val m = md.get(Keys.moduleID.key).get
    val a = md.get(Keys.artifact.key).get
    s"   ${m.organization}:${m.name}:${a.extension}:${m.revision}"
  }
  lines.sorted.foreach(l => IO.append(f, s"$l\n"))
  IO.append(f, "\n")
  f
}