The target of this patch is exports such as:

```
module Foo ( T(A, B, C) ) where
```

Essentially this patch makes sure that we use the correct lookup functions in order
to lookup the names in parent-children export lists. This change
highlighted the complexity of this small part of GHC which accounts for
the scale.

This change was motivated by wanting to
remove the `PatternSynonym` constructor from `Parent`. As with all these
things, it quickly spiraled out of control into a much larger refactor.

Reviewers: simonpj, goldfire, bgamari, austin

Subscribers: adamgundry, thomie

Differential Revision: https://phabricator.haskell.org/D2179

GHC Trac Issues: #11970