Add scripted scala3-instance-loader

sbt/src/sbt-test/project/scala3-instance-loader/build.sbt

@@ -0,0 +1,13 @@
+ThisBuild / scalaVersion := "3.0.0-M3"
+
+lazy val check = taskKey[Unit]("check the Scala 3 instance class loader")
+
+lazy val xsbtiClass = classOf[xsbti.compile.Compilers]
+
+check := {
+  val scala3Loader = scalaInstance.value.loader
+  assert(
+    scala3Loader.loadClass(xsbtiClass.getCanonicalName) == xsbtiClass,
+    "The Scala 3 instance classloader does not load the same `xsbti` classes than sbt"
+  )
+}

sbt/src/sbt-test/project/scala3-instance-loader/test

@@ -0,0 +1 @@
+> check