title: Issue with Gatsby's addThirdPartySchema
body: <p>I am trying to create a custom source plugin and am trying to use the <code>addThirdPartySchema</code> method, I tried with this simple example below but receive this error:</p>
  <pre><code>Error: Cannot create as TypeComposer the following value: Test.
  </code></pre>
  <pre class="lang-js prettyprint-override"><code>import * as graphql from &quot;graphql&quot;;
  import { SourceNodesArgs } from &quot;gatsby&quot;;
  
  export const sourceNodes = function sourceNodes(args: SourceNodesArgs) {
    const { addThirdPartySchema } = args.actions;
  
    const schema = new graphql.GraphQLSchema({
      query: new graphql.GraphQLObjectType({
        name: &quot;Test&quot;,
        fields: {
          test: {
            type: graphql.GraphQLString,
            resolve: () =&gt; &quot;hello&quot;,
          },
        },
      }),
    });
  
    addThirdPartySchema({
      schema,
    });
  };
  </code></pre>