printSubgraphSchema fails if using a schema transformer and extending an external type

[kal4l]

Issue Description

If your subgraph has a schema that references an external type, e.g.

type SubgraphType {
  id: ID!
  external: ExternalType
}

extend type ExternalType @key(fields: "id") {
  id: ID! @external
}

and you have a transformer applied to the schema, e.g.

const schemaText = fs.readFileSync(path.join(__dirname, 'schema.graphql')).toString()
const typeDefs = gql(schemaText)

let schema = buildSubgraphSchema([
  { typeDefs }
])

schema = basicSchemaTransformer(schema)

Then printSubgraphSchema will fail with the error

GraphQLError: Type ExternalType must define one or more fields.
    at Object.err (/Users/kgrant/git/print-subgraph-schema-error/node_modules/@apollo/federation-internals/src/error.ts:36:60)
    at Validator.addError (/Users/kgrant/git/print-subgraph-schema-error/node_modules/@apollo/federation-internals/src/validate.ts:138:45)
    at Validator.validateObjectOrInterfaceType (/Users/kgrant/git/print-subgraph-schema-error/node_modules/@apollo/federation-internals/src/validate.ts:168:12)
    at Validator.validate (/Users/kgrant/git/print-subgraph-schema-error/node_modules/@apollo/federation-internals/src/validate.ts:90:16)
    at validateSchema (/Users/kgrant/git/print-subgraph-schema-error/node_modules/@apollo/federation-internals/src/validate.ts:30:32)
    at Schema.validate (/Users/kgrant/git/print-subgraph-schema-error/node_modules/@apollo/federation-internals/src/definitions.ts:1606:42)
    at Subgraph.validate (/Users/kgrant/git/print-subgraph-schema-error/node_modules/@apollo/federation-internals/src/federation.ts:1766:19)
    at buildSubgraph (/Users/kgrant/git/print-subgraph-schema-error/node_modules/@apollo/federation-internals/src/federation.ts:1329:19)
    at printSubgraphSchema (/Users/kgrant/git/print-subgraph-schema-error/node_modules/@apollo/subgraph/src/printSubgraphSchema.ts:16:33)
    at Object.<anonymous> (/Users/kgrant/git/print-subgraph-schema-error/src/print-schema.ts:4:32) {
  message: '[<unnamed>] Type ExternalType must define one or more fields.',
  path: undefined,
  locations: undefined,
  extensions: { code: 'INVALID_GRAPHQL' }
}

Link to Reproduction

https://github.com/kal4l/print-subgraph-schema-error

Reproduction Steps

• See reproduction repo

Version

• "@apollo/subgraph": "2.5.7"