Using a view as an enum table
### Summary

I _think_ there is an issue with using a view as an [enum table](https://www.graphile.org/postgraphile/enums/#with-enum-tables).

A test database with the example code from the documentation works fine but if I try the same thing using views and smart tags to signal the FK it's not recognised.

### Steps to reproduce

The `private` schema contains the raw data tables, the `public` schema is what we expose to Postgraphile.

```sql
CREATE DATABASE test;
\c test

CREATE SCHEMA private;
CREATE EXTENSION pgcrypto;

/**
 * private schema
 *
 * enum table
 */
CREATE TABLE private.status_values (
  id serial primary key,
  type text unique not null,
  description text
);

CREATE TABLE private.foo (
  id serial primary key,
  external_id uuid unique not null default gen_random_uuid(),
  name text unique not null,
  status int4 not null references private.status_values (id)
);

/**
 * public view
 */
CREATE VIEW public.status_values AS (
  SELECT type, description
  FROM private.status_values
);
COMMENT ON VIEW public.status_values IS E'@enum';

CREATE VIEW public.foo AS (
  SELECT
    f.external_id "id",
    f.name,
    s.type "status"
  FROM private.foo f
  JOIN private.status_values s ON s.id = f.status
);
COMMENT ON VIEW public.foo IS E'@foreignKey (status) references public.status_values (type)';
```

### Expected results

Queries and mutations for `public.foo` should use the enum type for the `status` field.

### Actual results

![image](https://user-images.githubusercontent.com/103831/103697142-cc3ee380-4f9f-11eb-8b05-67f4f4f8deaf.png)

### Additional context

- macOS Catalina
- PostgreSQL 13.1
- Node v14.15.0
- Postgraphile 4.10.0

### Possible Solution

No suggestions for a fix but if you have any suggestions for how to troubleshoot this further I'm happy to take a stab. I tried various combinations of `DEBUG` configs but couldn't find a way to see what the schema generator thinks that relationship looks like...
