jsonnetfmt sorts imports weirdly

Let's say I have a seet of imports

```
service = import "./service.jsonnet"
serviceAddOn = import "./service-add-on.jsonnet"
```

Well, with jsonnefmt it comes out sorted as :

```
serviceAddOn = import "./service-add-on.jsonnet"
service = import "./service.jsonnet"
```

This is because "-" comes before "." in the ASCII alphabet. However, if you would simply drop the '.jsonnet' from each file name before sorting them, it would come out in what most people perceive as the correct order :

```
service = import "./service.jsonnet"
serviceAddOn = import "./service-add-on.jsonnet"
```