You can install typings (npm install -g typings) and then search for type definitions. For hapijs, it would be `typings search hapi`. Then you can install using `typings install dt~hapi --global --save`.
The list is truncated in web view on Github. If you use Visual Studio they all have nuget packages which lets you search for (and add them) easily to a project.
Edit: Thanks, I had a feeling there was a bug... it was just strange to see files from the top-level folder appear afterward, made it less obvious.