Make `required_cogs` key a dict in `info.json` files

[Jackenmen]

It's nothing very important since nothing even uses this key, but required_cogs key should be a dict mapping a cogname to repo URL, e.g.

{
    "required_cogs": {
        "adventure": "https://github.com/aikaterna/gobcog/"
    }
}

See docs for more info