7f26e50a

By: Greg Pfeil <greg@technomadic.org>

Fix accidental discarding of exports

The `traverse` means that if _any_ `exports` result in `mempty` from
`lookupExport`, then `mkExportItems` will return an empty list.

This crops up in at least the following cases:

| case                                        | source         |
|---------------------------------------------|----------------|
| `IEGroup`                                   | `-- * section` |
| `IEDocNamed`                                | `-- $named`    |
| `IEModuleContents` (whole-module re-export) | `module M`     |

In [Henforcer](https://github.com/flipstone/henforcer#readme), this makes
things like `maximumExportsPlusHeaderUndocumented = 0` trivially pass without
actually checking anything.

Using `concatMap` avoids the ×0 situation, so everything reports its results.

Total

2s

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