module: fix check for package.json at volume root

Fix package.json files at the volume root so that
when they contain {"type": "module"}, they behave
as documented, like such a package.json file in
any other folder.

Fixes: #33438

PR-URL: #33476
Reviewed-By: Guy Bedford <[email protected]>
Reviewed-By: Jan Krems <[email protected]>

Author: GeoffreyBooth

lib/internal/modules/cjs/loader.js

@@ -41,6 +41,7 @@ const {
   SafeMap,
   String,
   StringPrototypeIndexOf,
+  StringPrototypeLastIndexOf,
   StringPrototypeMatch,
   StringPrototypeSlice,
   StringPrototypeStartsWith,
@@ -286,12 +287,13 @@ function readPackageScope(checkPath) {
   const rootSeparatorIndex = checkPath.indexOf(path.sep);
   let separatorIndex;
   while (
-    (separatorIndex = checkPath.lastIndexOf(path.sep)) > rootSeparatorIndex
+    (separatorIndex = StringPrototypeLastIndexOf(checkPath, path.sep)) >=
+     rootSeparatorIndex
   ) {
     checkPath = checkPath.slice(0, separatorIndex);
     if (checkPath.endsWith(path.sep + 'node_modules'))
       return false;
-    const pjson = readPackage(checkPath);
+    const pjson = readPackage(checkPath + path.sep);
     if (pjson) return {
       path: checkPath,
       data: pjson