build.sbt

lazy val getAllDependencyProjects = taskKey[Seq[ProjectRef]](
  "依存してるprojectを再帰的に取得"
)

lazy val commonSettings = Def.settings(
  getAllDependencyProjects := {
    val map = buildDependencies.value.classpath
    def loop(root: ProjectRef): Seq[ProjectRef] = {
      root +: map(root).map(_.project).flatMap(loop)
    }
    loop(thisProjectRef.value).distinct
  },
  clean := {
    println(name.value + " をcleanします") // debug用
    clean.value
  },
  TaskKey[Unit]("cleanDependencyProjects", "依存してるprojectのすべてのcleanを実行") := Def.taskDyn{
    getAllDependencyProjects.value.map(_ / clean).join
  }.value
)

lazy val a = project.settings(commonSettings)
lazy val b = project.dependsOn(a).settings(commonSettings)
lazy val c = project.dependsOn(b).settings(commonSettings)
lazy val d = project.dependsOn(b).settings(commonSettings)