buildFederatedSchema throws when a directive uses an input type

I've tried replacing the input type with a scalar and an enum, they both work fine, it's only inputs that are crashing.

Reproducible example:
```js
const {gql} = require('apollo-server');
const {buildFederatedSchema} = require('@apollo/federation');

const typeDefs = gql`
directive @someDirective(input: SomeInput) on FIELD_DEFINITION

input SomeInput {
key: String
}
`;

buildFederatedSchema(typeDefs);
```

Package versions:
0.11.2 - @apollo/federation
2.9.15 - apollo-server
14.5.8 - graphql

Start with the provided reproduction and trace buildFederatedSchema, using the listed @apollo/federation, apollo-server, and graphql versions as context. Done means a directive whose argument uses an input type builds successfully without the reported crash.

Written by the indexing model from the issue text.