Hide __extras from extras on package_show

It is useless and adds a lot of noise:

From this:
```
[
    {
        "__extras": {
            "package_id": "59cf84f2-dbfe-4388-af12-d976a7439a76", 
            "revision_id": "62630fa3-05bb-4375-9f37-b9b40a9d1814"
        }, 
        "key": "created", 
        "value": "2008-10-07"
    }, 
    {
        "__extras": {
            "package_id": "59cf84f2-dbfe-4388-af12-d976a7439a76", 
            "revision_id": "62630fa3-05bb-4375-9f37-b9b40a9d1814"
        }, 
        "key": "source", 
        "value": ""
    }
]
```
To this:
```
[
    {
        "key": "created", 
        "value": "2008-10-07"
    }, 
    {
        "key": "source", 
        "value": ""
    }
]
```