Proposal: nested imports

This may cover common (as I think) situation when you need to use generated types but slightly change them. As an example — add directives (e.g. add `@auth` to mutations).

```
type Mutation @auth {
#import Mutation.createArticle from "src/schema/generated/mysql.graphql"
}
```
or/and

```
type Mutation {
createArticle @auth {
#import Mutation.createArticle.* from "src/schema/generated/mysql.graphql"
}
}
```

The proposal provides GraphQL SDL examples and references src/schema/generated/mysql.graphql, but names no implementation entry point or test. Start by locating the existing import handling and its tests, then confirm the supported nested-import behavior and completion criteria with maintainers.

Written by the indexing model from the issue text.