YAML attribute provider

I want to layout the attributes of the magento fixtures in YAML files. Customer and SImple products seems fine, however Address and Order will have dependencies and are built up in a more complex way.

How should we manage these dependencies?

Would an Order yaml file have everything explicitly defined? Or will there be a yaml file for customer, address, product and order, and have a flag in the order file to pull in these dependencies? 

```yaml
sales/order:
   customer: @customer
   ...
```

Would love input on where this should be going :+1: