[Serialization] Refactor logic deciding transitive module loading logic

Refactor and centralize the logic about how implementation-only and
package-only dependencies should be loaded.

lib/Serialization/SerializedModuleLoader.cpp

@@ -940,10 +940,11 @@ void swift::serialization::diagnoseSerializedASTLoadFailure(
     std::copy_if(
         loadedModuleFile->getDependencies().begin(),
         loadedModuleFile->getDependencies().end(), std::back_inserter(missing),
-        [&duplicates, &Ctx](const ModuleFile::Dependency &dependency) -> bool {
+        [&duplicates, &loadedModuleFile](
+            const ModuleFile::Dependency &dependency) -> bool {
           if (dependency.isLoaded() || dependency.isHeader() ||
-              (dependency.isImplementationOnly() &&
-               Ctx.LangOpts.DebuggerSupport)) {
+              loadedModuleFile->getTransitiveLoadingBehavior(dependency) !=
+                ModuleLoadingBehavior::Required) {
             return false;
           }
           return duplicates.insert(dependency.Core.RawPath).second;