Experimental Subschema empty interface

[tomgasson]

In order to work out what part of our very large schema is actually being used by our products, we've been exploring using the subschema extraction. Our intent is to identify dead / unused schema and make our breaking changes detection & test selection more fine-grained (specific to the consuming product)

We've run into a problem of producing invalid schema.

schema

type Query {
  search: Searchable
}

interface Searchable {
  title: String
  score: Float
}

type Article implements Searchable {
  title: String
  score: Float
  body: String
}

usage

graphql`
  query SearchQuery {
    search {
      ... on Article {
        body
      }
    }
  }
`;

output

type Article implements Searchable {
  body: String
}

type Query {
  search: Searchable
}

interface Searchable

which is invalid, since Searchable has no fields.

1. We can't remove Searchable because it appears as an output type of Query.search: Searchable and changing that would change the schema
2. We can't "fake" it with a dummy field because the Article would then need to implement that dummy field too
3. We can't use __typename (which is effectively what happens at runtime) because __ is reserved

This PR implements a "fix": If we find empty interfaces, we force their fields existence (and hence their implementing concrete types to have those fields). This means we do have "unused fields", but only for these cases.

Not ideal, however this only occurs 12 times for us so is acceptable, and I expect it's not encountered in Meta's schema if it's not yet addressed.