Harden definition of isScalaTrait

Exclude false positives such as `Lambda|` be requiring
that lambda traits are defined in the Scala package.

Author: odersky

src/dotty/tools/dotc/core/SymDenotations.scala

@@ -446,7 +446,7 @@ object SymDenotations {
 
     /** is this symbol a trait representing a type lambda? */
     final def isLambdaTrait(implicit ctx: Context): Boolean =
-      isClass && name.startsWith(tpnme.LambdaPrefix)
+      isClass && name.startsWith(tpnme.LambdaPrefix) && owner == defn.ScalaPackageClass
 
     /** Is this symbol a package object or its module class? */
     def isPackageObject(implicit ctx: Context): Boolean = {