JSON API exists for Snapshots but not for Packages

Calling `http --json https://www.stackage.org/lts-9.0` gets us a nice json answer with the snapshot content:
```
{
"packages": [
{
"isCore": false,
"name": "abstract-deque",
"synopsis": "Abstract, parameterized interface to mutable Deques.",
"version": "0.3"
},
...,
],
"snapshot": {
"created": "2017-07-26",
"ghc": "8.0.2",
"name": "lts-9.0"
}
}
```

But querying a single package (e.g. `http --json https://www.stackage.org/lts-9.0/package/base-4.9.1.0`) returns instead just the html page for the package.

It would be very nice (and consistent) to have json responses also on package pages, to enable programmatic navigation of Stackage.

If this makes sense I'd be very happy to put together a PR.