support project_dir_path in file_ref.real_path

https://github.com/CocoaPods/Xcodeproj/blob/29cd0821d47f864abbd1ca80f23ff2aded0adfed/lib/xcodeproj/project/object/helpers/groupable_helper.rb#L156

```
if ref.source_tree == "SOURCE_ROOT"
# fix bugs in https://github.com/CocoaPods/Xcodeproj/blob/29cd0821d47f864abbd1ca80f23ff2aded0adfed/lib/xcodeproj/project/object/helpers/groupable_helper.rb#L156
# support CMake generated projects with project_dir_path
file_path = (ref.project.project_dir + ref.project.root_object.project_dir_path + ref.path).to_s
else
file_path = ref.real_path.to_s
end
```

Start at lib/xcodeproj/project/object/helpers/groupable_helper.rb near line 156 and inspect how file_ref.real_path handles SOURCE_ROOT. Compare the proposed project_dir_path handling with the existing path resolution, using a CMake-generated project as the target case. Done means SOURCE_ROOT file paths resolve correctly when project_dir_path is set.

Written by the indexing model from the issue text.