unused var error is triggered when var appears in an input value

The unused variables check seems to be triggered when the variable appears as a value for a field on an input type. 

The following yields `Variable 'x' is undefined`.

```scala
query getZuckProfile($x: String) {
  search(
    pattern: {
      name: $x
    }
  ) {
    id
  }
}
```

See failing test in https://github.com/typelevel/grackle/pull/new/issue/743