When using the "extend" keyword only, types aren't automatically imported

Hey,

When using `extend` with a type, if it isn't anywhere in the schema without `extend`, input and return types used in its fields and the type itself aren't in the final schema unless explicitly imported.

This is a problem in federated services which sometimes only extend certain types.

```graphql
# query.graphql
extend type Query {
test: Test
}

type Test {
ok: Boolean
}
```

```graphql
# schema.graphql

# import Query from "query.graphql"
```

schema after importing `schema.graphql` (`Test` is absent):
```graphql
extend type Query {
test: Test!
}
```

Adding `type Query { anyField: AnyType }` anywhere produces a valid schema, but also drops the `extend` keyword

Other import methods:
```graphql
# import Query.* from "query.graphql"
```
Same result

```graphql
# import "query.graphql"
# import * from "query.graphql"
```
The `Query` type is not present at all (so in the exemple, it throws because it's an empty schema)

Start by reproducing the issue with the query.graphql and schema.graphql examples, then trace how importing an extend-only Query definition builds the final schema. Done means the imported schema retains Query, Test, and the field's input and return types without requiring an unrelated non-extended definition.

Written by the indexing model from the issue text.