pgQuery for `field` returning `enum` fails
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### Summary

I _think_ this is a bug. When we use `makeExtendSchema` to extend a type, and attempt to return an enum, we get the error:  `Error: @pgQuery(...) directive called with invalid arguments - for a table value, call it with 'source' for a scalar with 'fragment'!`

I'm having a hard time parsing this message, it's not very clear and could use some improvement. I also can't tell how it relates to my declaration below, or what the resolution is. Have we discovered a new bug?

### Steps to reproduce

We've got a TS enum defined as:

```ts
enum Severity {
  HIGH = 'HIGH',
  LOW = 'LOW',
  MEDIUM = 'MEDIUM',
  NEUTRAL = 'NEUTRAL',
  OK = 'OK'
}
```

And the same declared in GraphQL:

```graphql
    enum Severity {
      OK
      NEUTRAL
      LOW
      MEDIUM
      HIGH
    }
```
 
With a field declared as:

```
      treeShakeableLevel: Severity! @pgQuery(
        fragment: ${embed(
          (queryBuilder: any) =>
            pgSql.fragment`(
              select
              (case when (select
                (
                  coalesce(_extra->'module', _extra->'jsnext:main') is not null
                    or
                  (_extra->'type' is not null and (_extra->'type')::text = 'module')
                )
                from
                  collectors.npm_versions v
                where
                  v.npm_package_id = ${queryBuilder.getTableAlias()}.id
                and
                  v.version = ${queryBuilder.getTableAlias()}.latest_version
                and
                  v.is_deleted = false
              ) = true then ${pgSql.literal(Severity.OK)} else ${pgSql.literal(Severity.NEUTRAL )} end)
            )`
        )}
      )
```

### Expected results

Server starts and creates schema successfully

### Actual results

`Error: @pgQuery(...) directive called with invalid arguments - for a table value, call it with 'source' for a scalar with 'fragment'!`

### Additional context

Of note, if we change `Severity!` to `String!` the server starts without issue.
Please let me know if you need any additional information or schema, resolver code, etc.

### Possible Solution

n/a
