Correct discrepancies in the package interface file lookup.

Add a check for the client side flag which explcitly opts in to loading the package interface,
besides whether package-name is empty or in the same package.

rdar://131393508

Author: elsh

lib/Serialization/SerializedModuleLoader.cpp

@@ -618,9 +618,6 @@ std::string SerializedModuleBaseName::getName(file_types::ID fileTy) const {
 std::optional<std::string>
 SerializedModuleBaseName::getPackageInterfacePathIfInSamePackage(
     llvm::vfs::FileSystem &fs, ASTContext &ctx) const {
-  if (!ctx.LangOpts.EnablePackageInterfaceLoad)
-    return std::nullopt;
-
   std::string packagePath{
       getName(file_types::TY_PackageSwiftModuleInterfaceFile)};
 
@@ -667,8 +664,12 @@ SerializedModuleBaseName::findInterfacePath(llvm::vfs::FileSystem &fs,
   if (!fs.exists(interfacePath))
     return std::nullopt;
 
-  // If there is a package name, try look for the package interface.
-  if (!ctx.LangOpts.PackageName.empty()) {
+  // If there is a package name, try to look for the package interface,
+  // which can only be loaded if -experimental-package-interface-load
+  // is passed to the client, so fall back to private/public interface
+  // if not.
+  if (!ctx.LangOpts.PackageName.empty() &&
+      ctx.LangOpts.EnablePackageInterfaceLoad) {
     if (auto maybePackageInterface =
             getPackageInterfacePathIfInSamePackage(fs, ctx))
       return *maybePackageInterface;