support for structured GraphQL Schema

Hi, it's a feature request not a bug.

I think GraphQL schema could be more flexible than now.
For now, I have to define all GraphQL schema and resolver to `schema.graphql`, `resolvers.js`. I know `resolvers` are more options but not enough.

My proper structure for GraphQL is like this. each entry has own definitions.

```
src/
├── definitions
│   ├── groot
│   │   ├── groot.resolvers.js
│   │   └── groot.graphql
│   └── hello
│   ├── hello.resolvers.js
│   └── hello.graphql
└── index.js

3 directories, 5 files

```

However, this structure is not worked with `graphqlpack`. [code sandbox](https://codesandbox.io/s/910qvoo4or) has following error message.

```
Error: Couldn't find any resolvers. Please add resolvers to your src/resolvers.js
at Module../node_modules/graphpack/lib/server.js
```

How about your thought on above GraphQL schema structure support?
I think `merge-graphql-schemas` can make things works easily. [this repository](https://github.com/vreality64/graphql-example) will show you how to use the `merge-graphql-schemas` and what the benefits are. 😃