The AvailTC was not be upheld for explicit export module
export lists when the module contains associated data families.

    module A (module A) where
    class    C a  where { data T a }
    instance C () where { data T () = D }

Used to (incorrectly) report avails as `[C{C, T;}, T{D;}]`. Note that
although `T` is exported, the avail where it is the parent does _not_
list it as its first element. This avail is now correctly listed as
`[C{C, T;}, T{T, D;}]`.

This was induces a [crash in Haddock][0].

See #16077.

[0]: https://github.com/haskell/haddock/issues/979