Follow up to #6918: Refine elabStanzasRequested

```haskell
-- TODO: The 'Bool' here should be refined into an ADT with three
-- cases: NotRequested, ExplicitlyRequested and
-- ImplicitlyRequested. A stanza is explicitly requested if
-- the user asked, for this *specific* package, that the stanza
-- be enabled; it's implicitly requested if the user asked for
-- all global packages to have this stanza enabled. The
-- difference between an explicit and implicit request is
-- error reporting behavior: if a user asks for tests to be
-- enabled for a specific package that doesn't have any tests,
-- we should warn them about it, but we shouldn't complain
-- that a user enabled tests globally, and some local packages
-- just happen not to have any tests. (But perhaps we should
-- warn if ALL local packages don't have any tests.)
```