sbt-dotty: Use default dependency resolution

In particular, this should use coursier instead of ivy unless someone
forced the use of ivy. And we shouldn't have to worry about
inter-project dependencies in the dotty build because of
coursier/sbt-coursier#101.

Author: smarter

sbt-dotty/src/dotty/tools/sbtplugin/DottyPlugin.scala

@@ -302,15 +302,15 @@ object DottyPlugin extends AutoPlugin {
       // ... instead, we'll fetch the compiler and its dependencies ourselves.
       scalaInstance := Def.taskDyn {
         if (isDotty.value)
-          dottyScalaInstanceTask("dotty-compiler")
+          dottyScalaInstanceTask("dotty-compiler", dependencyResolution.value)
         else
           Def.valueStrict { scalaInstance.taskValue }
       }.value,
 
       // We need more stuff on the classpath to run the `doc` task.
       scalaInstance in doc := Def.taskDyn {
         if (isDotty.value)
-          dottyScalaInstanceTask("dotty-doc")
+          dottyScalaInstanceTask("dotty-doc", dependencyResolution.value)
         else
           Def.valueStrict { (scalaInstance in doc).taskValue }
       }.value,
@@ -404,16 +404,11 @@ object DottyPlugin extends AutoPlugin {
   }
 
   /** Create a scalaInstance task that uses Dotty based on `moduleName`. */
-  def dottyScalaInstanceTask(moduleName: String): Initialize[Task[ScalaInstance]] = Def.task {
-    val ivyConfig0 = Classpaths.mkIvyConfiguration.value
-    // When compiling non-bootstrapped projects in the build of Dotty itself,
-    // dependency resolution might pick a local project which is not what we
-    // want. We avoid this by dropping the inter-project resolver.
-    val ivyConfig1 = ivyConfig0.withResolvers(ivyConfig0.resolvers.filter(_.name != "inter-project"))
+  def dottyScalaInstanceTask(moduleName: String, dependencyRes: DependencyResolution): Initialize[Task[ScalaInstance]] = Def.task {
     val updateReport =
       fetchArtifactsOf(
         scalaOrganization.value %% moduleName % scalaVersion.value,
-        ivy.IvyDependencyResolution(ivyConfig1),
+        dependencyRes,
         scalaModuleInfo.value,
         updateConfiguration.value,
         (unresolvedWarningConfiguration in update).value,