Limit scope of `ExportedName`/`ImportedName` case distinction

[edsko]

This distinction was introduced in #1319, but was used everywhere we previously used Hs.Name. In hindsight, this was probably not necessary. Not entirely sure yet what the best solution here; needs a bit of thought.

[edsko]

#1404 addresses this to some degree by introducing ExportedName, but we should think about this a bit more carefully.