[Dependency Scanning] Modernize and revive the module dependencies cache serialization format

This format has gotten stale and been not in use for many months. This commit restores primary functionality of the format.

include/swift/AST/ModuleDependencies.h

@@ -769,6 +769,15 @@ public:
   setLinkLibraries(const ArrayRef<LinkLibrary> linkLibraries) {
     storage->linkLibraries.assign(linkLibraries.begin(), linkLibraries.end());
   }
+  
+  const ArrayRef<std::string> getAuxiliaryFiles() const {
+    return storage->auxiliaryFiles;
+  }
+  
+  void
+  setAuxiliaryFiles(const ArrayRef<std::string> auxiliaryFiles) {
+    storage->auxiliaryFiles.assign(auxiliaryFiles.begin(), auxiliaryFiles.end());
+  }
 
   bool isStaticLibrary() const {
     if (auto *detail = getAsSwiftInterfaceModule())