I wouldn't call it *good* yet, but this will do for now.
- `RetrieveRegularNARSink` renamed to `RegularFileSink` and moved
accordingly because it actually has nothing to do with NARs in
particular.
- its `fd` field is also marked private
- `copyRecursive` introduced to dump a `SourceAccessor` into a
`ParseSink`.
- `NullParseSink` made so `ParseSink` no longer has sketchy default
methods.
This was done while updating #8918 to work with the new
`SourceAccessor`.