libutil: Fix copyRecursive and use for nix flake clone

The use of sourceToSink is an unnecessary serialization bottleneck.
While we are at it, fix the copyRecursive implementation to actually copy
the whole directory. It wasn't used for anything prior, but now it has a use
and accompanying tests for flake clone.

src/libfetchers/fetchers.cc

@@ -495,9 +495,9 @@ void InputScheme::clone(
 
     Activity act(*logger, lvlTalkative, actUnknown, fmt("copying '%s' to %s...", input2.to_string(), destDir));
 
-    auto source = sinkToSource([&](Sink & sink) { accessor->dumpPath(CanonPath::root, sink); });
-
-    restorePath(destDir, *source);
+    RestoreSink sink(/*startFsync=*/false);
+    sink.dstPath = destDir;
+    copyRecursive(*accessor, CanonPath::root, sink, CanonPath::root);
 }
 
 std::optional<ExperimentalFeature> InputScheme::experimentalFeature() const

src/libutil/fs-sink.cc

@@ -37,7 +37,6 @@ void copyRecursive(SourceAccessor & accessor, const CanonPath & from, FileSystem
         sink.createDirectory(to, [&](FileSystemObjectSink & dirSink, const CanonPath & relDirPath) {
             for (auto & [name, _] : accessor.readDirectory(from)) {
                 copyRecursive(accessor, from / name, dirSink, relDirPath / name);
-                break;
             }
         });
         break;