federation spec: types with all fields marked @external

Suppose you have some service which needs to return a type which it does not implement nor extend. For example:
```graphql
extend type Foo @key(fields: "id") {
id: ID @external
}

type Bar {
relatedFoo: Foo
}
```

In this case, the only thing this service ever needs to do with `Foo` is to return a Foo containing only an ID.

However, the [spec says](https://www.apollographql.com/docs/apollo-server/federation/federation-spec/#resolve-requests-for-entities):
> To implement the `_Entity` union, each type annotated with `@key` should be added to the `_Entity` union

Thus `Foo` does have to get added to `_Entity`, which means in principle the service is supposed to able to handle an `_entities` query for one. But I don't see why: we should never be asked to look up a `Foo` by key, since we only return them.

(A special cases is where there's another field `Foo.name @external`, say, and `relatedFoo` has `@provides(fields: "name")`. But the result is the same: we should never be asked to look up a Foo, even though we may return some fields of one.)

So: can we exclude types with all fields `@external` from `_Entity`? Or is there some way such entities can be requested?

(For context, this came up in `gqlgen`, a Go GraphQL server with federation support: https://github.com/99designs/gqlgen/issues/1076.)

Start with the federation specification's “Resolve requests for entities” section and compare it with the gqlgen issue linked in the report. Determine whether a type whose fields are all @external can be omitted from _Entity or can otherwise be requested; done means the specification question has a clear, documented resolution.

Written by the indexing model from the issue text.