Add dotty build settings

Author: giabao

build.sbt

@@ -23,7 +23,16 @@ developers := List(
   )
 )
 
-crossScalaVersions := List("2.10.7", "2.11.12", "2.12.12", "2.13.3")
+crossScalaVersions := List("2.10.7", "2.11.12", "2.12.12", "2.13.3", "0.26.0-RC1")
+
+/** Add src/main/scala-{2|3} to Compile / unmanagedSourceDirectories */
+Compile / unmanagedSourceDirectories ++= {
+  val sourceDir = (Compile / sourceDirectory).value
+  CrossVersion.partialVersion(scalaVersion.value).map {
+    case (0 | 3, _) => sourceDir / "scala-3"
+    case (n, _) => sourceDir / s"scala-$n"
+  }
+}
 
 libraryDependencies ++= Seq(
   "org.scalatest" %% "scalatest" % "3.1.3",

project/plugins.sbt

@@ -3,3 +3,5 @@ addSbtPlugin("com.jsuereth" % "sbt-pgp" % "1.1.1")
 addSbtPlugin("com.geirsson" % "sbt-ci-release" % "1.2.2")
 
 addSbtPlugin("com.typesafe.sbt" % "sbt-osgi" % "0.9.4")
+
+addSbtPlugin("ch.epfl.lamp" % "sbt-dotty" % "0.4.1")