Fix broken migration tests on main (#6986)

These were broken since we don't run migration tests by default due to
performance concerns, so just fixing them post-breakage

Author: lihaoyi

core/api/src/mill/api/internal/SubfolderModule.scala

@@ -15,7 +15,7 @@ object SubfolderModule {
     val prefix = mill.constants.CodeGenConstants.globalPackagePrefix
     val stripped = pkg.stripPrefix(prefix)
     if (stripped.isEmpty || stripped == ".") Seq.empty
-    else stripped.stripPrefix(".").split('.').toSeq
+    else stripped.stripPrefix(".").split('.').map(scala.reflect.NameTransformer.decode).toSeq
   }
 }