Generate input types

A query
```graphql
query getFoo($input: MyInput) {
...
}
```
where `MyInput` is
```graphql
input MyInput {
foo: Int
}
```
should generate
```haskell
data MyInput = MyInput
{ _foo :: Int
}

data GetFooArgs = GetFooArgs
{ _input :: MyInput
}
```

Start by locating the GraphQL query and input-type generation paths in the repository, then trace how the shown MyInput and GetFooArgs types are produced. The work is complete when a query using an input variable generates the Haskell declarations shown in the issue, including the nested input field and argument type.

Written by the indexing model from the issue text.