New broken prelude.resolvers.go file with v0.17.0

### What happened?

Since v0.17.0, running gqlgen throws some errors:
```
Error: validation failed: packages.Load: /graph/prelude.resolvers.go:23:41: __SchemaResolver not exported by package generated
Error: /graph/prelude.resolvers.go:26:39: __TypeResolver not exported by package generated
```

It generates a prelude.resolvers.go file that is not generated in v0.16.0, which seems to have some errors in it.

### What did you expect?

gqlgen v0.17.0 generates the same proper files as gqlgen v0.16.0, without errors.

### Minimal graphql.schema and models to reproduce
```graphql
type Object {
id: ID!
text: String!
}

input NewObject {
text: String!
}

type Mutation {
createObject(input: NewObject!): Object!
}
```

### versions
- `go run github.com/99designs/gqlgen version`: `v0.17.0` (latest as of writing this issue)
- `go version`: `go version go1.16.1 linux/amd64`

这个 issue 涉及 gqlgen v0.17.0 中一个损坏的生成文件 prelude.resolvers.go。首先检查提供的 GraphQL schema 以及 graph/ 目录中的生成代码。查看 gqlgen 的代码生成逻辑，尤其是 prelude resolver 的生成部分。检查生成的 package 中是否有导出的类型 __SchemaResolver 和 __TypeResolver。运行 gqlgen 命令以复现错误，并查看生成的 prelude.resolvers.go 文件。修复可能需要更新 code generator，以正确导出这些类型，或者调整生成的代码。

由索引模型根据 Issue 内容生成。