Resource transformation key conflict when multiple languages in multihost mode and each with its own content dir

Which would mean that:

```
{{ .Resources.GetMatch "*.css" | minify }}
```

Would give a cached and wrong result for the other languages if the CSS filename is the same.

This would not be the case if you have all content living in the same content folder, e.g.:

```bash
index.md
style.en.css
style.fr.css
```

