[Serialization] Do not accept packages without a name

Realign the module loading behavior with the one of the package
access-level. If the package name is an empty string, don't accept other
modules with an empty package name as being part of the same module and
don't load package dependencies in such a case.

lib/Serialization/ModuleFileSharedCore.cpp

@@ -1712,7 +1712,7 @@ ModuleFileSharedCore::getTransitiveLoadingBehavior(
   if (dependency.isPackageOnly()) {
     // Package dependencies are usually loaded only for import from the same
     // package.
-    if (packageName == getModulePackageName()) {
+    if (!packageName.empty() && packageName == getModulePackageName()) {
       return ModuleLoadingBehavior::Required;
     } else if (debuggerMode) {
       return ModuleLoadingBehavior::Optional;