Gateway does not merge Query, Mutation and Subscription properly if `extend` is not used

[PacoDu]

The gateway doesn't merge services schema correctly if extend is not used. The following schemas:

type Notification {
  id: ID!
  message: String
}

type Query {
  notifications: [Notification]
}

type User {
  id: ID!
  name: String
}

type Query {
  users: [User]
}

Produces:

directive @external on FIELD_DEFINITION
directive @requires(fields: _FieldSet!) on FIELD_DEFINITION
directive @provides(fields: _FieldSet!) on FIELD_DEFINITION
directive @key(fields: _FieldSet!) on OBJECT | INTERFACE
directive @extends on OBJECT | INTERFACE

scalar _Any
scalar _FieldSet

type User {
  id: ID!
  name: String
}

type Query {
  notifications: [Notification]
}

type Notification {
  id: ID!
  message: String
}

Current workaround: always use extend type Query/Mutation/Subscription for federated services. Related to: #262