Avoid round-trip through sourceModule in scalacLinkedClass.

The round-trip is useless, and causes issues with some
Scala2-emitted modules that do not have module classes (seems to be
related to modules inside traits).

Author: sjrd

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

@@ -1247,7 +1247,7 @@ object SymDenotations {
 
     final def scalacLinkedClass(using Context): Symbol =
       if (this.is(ModuleClass)) companionNamed(effectiveName.toTypeName)
-      else if (this.isClass) companionNamed(effectiveName.moduleClassName).sourceModule.moduleClass
+      else if (this.isClass) companionNamed(effectiveName.moduleClassName)
       else NoSymbol
 
     /** Find companion class symbol with given name, or NoSymbol if none exists.