Report invalid index

It is a common pattern to have an index file that exports all the siblings, e.g.

Suppose the following directory structure.

```
./src/loaders/
├── BadgeByBadgeIdLoader.js
├── BadgesByPostIdLoader.js
├── BlogByBlogIdLoader.js
├── PostsByBlogIdLoader.js
├── UserByUserIdLoader.js
└── index.js
```

Then the `./index.js` would contain:

``` js
export BadgeByBadgeIdLoader from './BadgeByBadgeIdLoader';
export BadgesByPostIdLoader from './BadgesByPostIdLoader';
export BlogByBlogIdLoader from './BlogByBlogIdLoader';
export PostsByBlogIdLoader from './PostsByBlogIdLoader';
export UserByUserIdLoader from './UserByUserIdLoader';
```

This rule would check all `./index.js` files that only have import/export statements.

If a matched file does not list one of the sibling files in the directory, it would report a problem.

Start with the proposed ./index.js pattern and review the repository's existing import/export rule entry points and tests. The change is complete when index files containing only import/export statements report missing sibling files while preserving the stated sibling-export pattern.

Written by the indexing model from the issue text.