Module structure proposal

For inspiration I looked at the [SoundCloud client library](https://bitbucket.org/sebasmagri/hscd/src/).

I propose we take `Network.Guardian.ContentApi` as the top module, and put the data types and their Aeson instances in a flat structure beneath that, e.g.

```
Network.Guardian.ContentApi
Network.Guardian.ContentApi.Tag
Network.Guardian.ContentApi.Section
Network.Guardian.ContentApi.{etc...}
```

For the sake of this PR, I've just moved all the existing data types into the `Tag` module for now, but there are probably things that can be extracted once we implement more than tag search.

I know in the Scala library we have `openplatform` in the module names, but I avoided that because it seems like filler which doesn't say anything helpful about what's in the module.