Make RewritingSink accept a map of rewrites

Giving it the same semantics as `rewriteStrings`.
Also add some tests for it

src/libstore/path-references.cc

@@ -0,0 +1,73 @@
+#include "path-references.hh"
+#include "hash.hh"
+#include "util.hh"
+#include "archive.hh"
+
+#include <map>
+#include <cstdlib>
+#include <mutex>
+#include <algorithm>
+
+
+namespace nix {
+
+
+PathRefScanSink::PathRefScanSink(StringSet && hashes, std::map<std::string, StorePath> && backMap)
+    : RefScanSink(std::move(hashes))
+    , backMap(std::move(backMap))
+{ }
+
+PathRefScanSink PathRefScanSink::fromPaths(const StorePathSet & refs)
+{
+    StringSet hashes;
+    std::map<std::string, StorePath> backMap;
+
+    for (auto & i : refs) {
+        std::string hashPart(i.hashPart());
+        auto inserted = backMap.emplace(hashPart, i).second;
+        assert(inserted);
+        hashes.insert(hashPart);
+    }
+
+    return PathRefScanSink(std::move(hashes), std::move(backMap));
+}
+
+StorePathSet PathRefScanSink::getResultPaths()
+{
+    /* Map the hashes found back to their store paths. */
+    StorePathSet found;
+    for (auto & i : getResult()) {
+        auto j = backMap.find(i);
+        assert(j != backMap.end());
+        found.insert(j->second);
+    }
+
+    return found;
+}
+
+
+std::pair<StorePathSet, HashResult> scanForReferences(
+    const std::string & path,
+    const StorePathSet & refs)
+{
+    HashSink hashSink { htSHA256 };
+    auto found = scanForReferences(hashSink, path, refs);
+    auto hash = hashSink.finish();
+    return std::pair<StorePathSet, HashResult>(found, hash);
+}
+
+StorePathSet scanForReferences(
+    Sink & toTee,
+    const Path & path,
+    const StorePathSet & refs)
+{
+    PathRefScanSink refsSink = PathRefScanSink::fromPaths(refs);
+    TeeSink sink { refsSink, toTee };
+
+    /* Look for the hashes in the NAR dump of the path. */
+    dumpPath(path, sink);
+
+    return refsSink.getResultPaths();
+}
+
+}