"Expected a PgSelectSingleStep [...] however we received 'Access'" from custom plugin
### Summary

An unexpected error is returned for the following plugin.

Discord discussion linked here: https://discord.com/channels/489127045289476126/498852330754801666/1280561965109874688

<!-- A brief summary of the problem. -->

### Steps to reproduce

Create a plugin:

```ts
export const TestPlugin = makeExtendSchemaPlugin((build) => {
  const { users } = build.input.pgRegistry.pgResources;

  return {
    typeDefs: gql`
      type MyCustomResponse {
        user: User
        duplicateUser: User
      }

      extend type Query {
        getMyself: MyCustomResponse
      }
    `,

    plans: {
      Query: {
        getMyself: () => {
          const $userId = context().get("userId");
          return object({
            user: users.get({ id: $userId }),
            duplicateUser: users.get({ id: $userId }),
          });
        },
      },
    },
  };
});
```

And write the following query:

```graphql
{
  getMyself {
    user {
      id
    }
    duplicateUser {
      id
    }
  }
}
```

### Expected results

We receive a GraphQL response containing the user and duplicateUser fields with the same id.

### Actual results

Error:

```
Query planning error: The step returned by 'getMyself.duplicateUser' is not compatible with the GraphQL object type 'User': Expected a PgSelectSingleStep, PgInsertSingleStep, PgUpdateSingleStep or PgDeleteSingleStep, however we received 'Access{1}➊<21.duplicateUser>[25]'.",
```

### Additional context

Adding an explicit plan resolver for the `duplicateUser` field resolves this:

```ts
MyCustomResponse: {
  duplicateUser($obj) {
    return $obj.get('duplicateUser');
  }
}
```

Also, removing the `duplicateUser` field from the GraphQL query allows the `user` field to be returned correctly.
However, querying only `duplicateUser` returns the same error.

### Possible Solution

Workaround is given above, but this appears to be a bug within Postgraphile/Grafast itself.

