Convert relative file's path to lowercase when looking for relative

Not sure this should be done in `builder2` or in `manifest.js`, but to demonstrate the bug.

Also to avoid repeating `toLowerCase` we can possibly re-write them to something like:

```javascript
  for (var i = 0; i < files.length; i++) {
    var f = files[i];
    var p = f.path.toLowerCase();
    for (var j = 0; j < extensions.length; j++) {
      // check by adding extensions
      if (p === path + extensions[j]) return f.name;
    }
    // check by removing extensions
    if (p.replace(/\.\w+$/, '') === path) return f.name;
  }
````