Make ParseSink a bit better

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`.

src/libstore/export-import.cc

@@ -65,7 +65,7 @@ StorePaths Store::importPaths(Source & source, CheckSigsFlag checkSigs)
         /* Extract the NAR from the source. */
         StringSink saved;
         TeeSource tee { source, saved };
-        ParseSink ether;
+        NullParseSink ether;
         parseDump(ether, tee);
 
         uint32_t magic = readInt(source);