Rename the api file names

[diegohaz]

It should look more like:

api
├── user
│   ├── api.js
│   ├── api.test.js
│   ├── controller.js
│   ├── index.js
│   ├── model.js
│   └── model.test.js
└── index.js

Where api/user/api.js is the current api/user/user.router.js. I think it still makes sense while gives a better tree (since api comes alphabetically first).

The api/index.js file is the current routes.js file.

Also, this makes easier to change the endpoint names as one no longer needs to update the name of each file.

The api/user/index.js file should look like:

export api from './api'
export controller from './controller'
export model, { schema } from './model'