Provide support for localised resources

An example project, created with regular Xcode, contains two localized `Localizable.strings` files:


When inspecting the project with `xcodeproj show ...`, it will list localized files like this:

```
- Resources:
- Localizable.strings:
- en
- nl
```

The underlying `Localizable.strings`-files are actually in two folders called en.lproj and nl.lproj:


When trying to mimic this setup, with the same underlying paths I add them like this:

```
project.new_file('Resources/en.lproj/Localizable.strings', 'Resources/Localizable.strings/en')
```

But the result is different from the way Xcode treats these special cases:


Would you know how to add these files, so Xcode treats them the same way, as when you add them the old-fashioned way?

Start by tracing the `project.new_file` entry point for paths such as `Resources/en.lproj/Localizable.strings` and the `Resources/Localizable.strings/en` group. Compare its project representation with the localized resources shown by `xcodeproj show`; done means both localized files are represented the way Xcode creates them.

Written by the indexing model from the issue text.