gqlgen doesn't generate code by contract with @specifiedBy or @deprecated

### What happened?

https://github.com/krupyansky/gqlgen-bug

Run `go run graph/generate.go`

After I can see error "failed to load schema: graph/schema.graphqls:23: Undefined directive shareable."

If I remove "@specifiedBy" from graph/schema.graphqls, or remove "@deprecated", or move declaration of scalar on the top, than I can see generated files without error.

### What did you expect?

I expect generated code without removing or moving declarations, because my contract is right.

### Minimal graphql.schema and models to reproduce

https://github.com/krupyansky/gqlgen-bug

### versions

https://github.com/krupyansky/gqlgen-bug

问题位于 gqlgen 代码生成器中。首先检查 schema 解析逻辑，相关代码很可能位于 `graphql/introspection` 或 `codegen` 包中。该 bug 与指令顺序（`@specifiedBy`、`@deprecated`）以及标量声明有关。运行链接仓库中的复现用例，然后跟踪错误 'Undefined directive shareable.'，以了解指令验证如何与标量定义交互。

由索引模型根据 Issue 内容生成。