Fix cyclic reference in Trees.scala

The previous commit added _root_.scala.Serializable as a parent to every
object, this lead to a cyclic reference manifesting itselfs as "not
found: type Type" in Trees.scala that we worked around by replace `Type`
with `Types.Type`. This commit fixes this properly by adding a shortcut
in typedIdent when typing `_root_` which avoids forcing imports.

Author: smarter

compiler/src/dotty/tools/dotc/ast/Trees.scala

@@ -887,8 +887,7 @@ object Trees {
 
   // ----- Generic Tree Instances, inherited from `tpt` and `untpd`.
 
-  // FIXME: Work around cyclic reference by writing `Types.Type` instead of `Type`
-  abstract class Instance[T >: Untyped <: Types.Type] { inst =>
+  abstract class Instance[T >: Untyped <: Type] { inst =>
 
     type Tree = Trees.Tree[T]
     type TypTree = Trees.TypTree[T]

compiler/src/dotty/tools/dotc/typer/Typer.scala

@@ -361,6 +361,11 @@ class Typer extends Namer
       if (untpd.isVarPattern(tree) && name.isTermName)
         return typed(desugar.patternVar(tree), pt)
     }
+    // Shortcut for the root package, this is not just a performance
+    // optimization, it also avoids forcing imports thus potentially avoiding
+    // cyclic references.
+    if (name == nme.ROOTPKG)
+      return tree.withType(defn.RootPackage.termRef)
 
     val rawType = {
       val saved1 = unimported