Switch to build.sbt

Using the same technique than scala-js where we just forward to
Build.scala: scala-js/scala-js#2312

build.sbt

@@ -0,0 +1,21 @@
+val  dotty = Build.dotty
+val `dotty-bootstrapped` = Build.`dotty-bootstrapped`
+val `dotty-interfaces` = Build.`dotty-interfaces`
+val `dotty-bot` = Build.`dotty-bot`
+val  dottyCompilerSettings = Build.dottyCompilerSettings
+val `dotty-compiler` = Build.`dotty-compiler`
+val `dotty-compiler-bootstrapped` = Build.`dotty-compiler-bootstrapped`
+val `dotty-bin-tests` = Build.`dotty-bin-tests`
+val  dottyLibrarySettings = Build.dottyLibrarySettings
+val `dotty-library` = Build.`dotty-library`
+val `dotty-library-bootstrapped` = Build.`dotty-library-bootstrapped`
+val  cleanSbtBridge = Build.cleanSbtBridge
+val `dotty-sbt-bridge` = Build.`dotty-sbt-bridge`
+val  sjsSandbox = Build.sjsSandbox
+val `dotty-bench` = Build.`dotty-bench`
+val `scala-library` = Build.`scala-library`
+val `scala-compiler` = Build.`scala-compiler`
+val `scala-reflect` = Build.`scala-reflect`
+val `scalap` = Build.`scalap`
+
+inThisBuild(Build.thisBuildSettings)

project/Build.scala

@@ -10,7 +10,7 @@ import org.scalajs.sbtplugin.ScalaJSPlugin
 import org.scalajs.sbtplugin.ScalaJSPlugin.autoImport._
 import sbt.Package.ManifestAttributes
 
-object DottyBuild extends Build {
+object Build {
 
   val scalacVersion = "2.11.5" // Do not rename, this is grepped in bin/common.
 
@@ -65,26 +65,25 @@ object DottyBuild extends Build {
     "dottydoc dependencies, should be moved to a dottydoc sbt subproject eventually"
   )
 
-  override def settings: Seq[Setting[_]] = {
-    super.settings ++ Seq(
-      scalaVersion in Global := scalacVersion,
-      version in Global := dottyVersion,
-      organization in Global := dottyOrganization,
-      organizationName in Global := "LAMP/EPFL",
-      organizationHomepage in Global := Some(url("http://lamp.epfl.ch")),
-      homepage in Global := Some(url("https://github.com/lampepfl/dotty")),
-
-      // scalac options
-      scalacOptions in Global ++= Seq(
-        "-feature",
-        "-deprecation",
-        "-encoding", "UTF8",
-        "-language:existentials,higherKinds,implicitConversions"
-      ),
-
-      javacOptions in Global ++= Seq("-Xlint:unchecked", "-Xlint:deprecation")
-    )
-  }
+  // Used in build.sbt
+  val thisBuildSettings = Seq(
+    scalaVersion in Global := scalacVersion,
+    version in Global := dottyVersion,
+    organization in Global := dottyOrganization,
+    organizationName in Global := "LAMP/EPFL",
+    organizationHomepage in Global := Some(url("http://lamp.epfl.ch")),
+    homepage in Global := Some(url("https://github.com/lampepfl/dotty")),
+
+    // scalac options
+    scalacOptions in Global ++= Seq(
+      "-feature",
+      "-deprecation",
+      "-encoding", "UTF8",
+      "-language:existentials,higherKinds,implicitConversions"
+    ),
+
+    javacOptions in Global ++= Seq("-Xlint:unchecked", "-Xlint:deprecation")
+  )
 
   /** Enforce 2.11.5. Do not let it be upgraded by dependencies. */
   private val overrideScalaVersionSetting =