Fixes an issue where directories generated for API items configured to
yield directory-wise hierarchy (via the `hierarchyBoundaries` option)
would be generated with names that differed from their corresponding
document names.

- Longer term, it would be nice to make the relationship between
directory names and document names less intertwined, but for now there
are aspects of the system that rely on the two being the same, and this
invariant was being violated. So, for now, this is considered a bug fix.

See updated test snapshots for examples of the impact.


[AB#24263](https://dev.azure.com/fluidframework/235294da-091d-4c29-84fc-cdfc3d90890b/_workitems/edit/24263)