Fix #3000: Do not flatten across packages

compiler/src/dotty/tools/dotc/transform/Flatten.scala

@@ -27,7 +27,20 @@ class Flatten extends MiniPhaseTransform with SymTransformer { thisTransform =>
     else ref
   }
 
-  private val liftedDefs = new mutable.ListBuffer[Tree]
+  private var liftedDefs = new mutable.ListBuffer[Tree]
+  private var liftedDefsQueue = List.empty[mutable.ListBuffer[Tree]]
+
+  override def prepareForPackageDef(tree: PackageDef)(implicit ctx: Context) = {
+    liftedDefsQueue = liftedDefs :: liftedDefsQueue
+    liftedDefs = new mutable.ListBuffer[Tree]
+    this
+  }
+
+  override def transformPackageDef(tree: PackageDef)(implicit ctx: Context, info: TransformerInfo) = {
+    liftedDefs = liftedDefsQueue.head
+    liftedDefsQueue = liftedDefsQueue.tail
+    tree
+  }
 
   private def liftIfNested(tree: Tree)(implicit ctx: Context, info: TransformerInfo) =
     if (ctx.owner is Package) tree

tests/pos/i3000.scala

@@ -0,0 +1,5 @@
+class Foo {
+  new Object { }
+}
+
+package bar { }

tests/run/i3000b.check

@@ -0,0 +1,2 @@
+Foo$$anon$1
+bar.Bar$$anon$2

tests/run/i3000b.scala

@@ -0,0 +1,18 @@
+object Test {
+  def main(args: Array[String]): Unit = {
+    new Foo
+    new bar.Bar
+  }
+}
+
+class Foo {
+  new Object { println(this.getClass.getName) }
+}
+
+package bar {
+  package baz { }
+  class Bar {
+    new Object { println(this.getClass.getName) }
+  }
+  package quxx { }
+}