nix-store -r: Handle symlinks to store paths

Fixes #3270.

src/libstore/path.cc

@@ -96,4 +96,19 @@ StorePathSet singleton(const StorePath & path)
     return res;
 }
 
+std::pair<std::string_view, StringSet> parsePathWithOutputs(std::string_view s)
+{
+    size_t n = s.find("!");
+    return n == s.npos
+        ? std::make_pair(s, std::set<string>())
+        : std::make_pair(((std::string_view) s).substr(0, n),
+            tokenizeString<std::set<string>>(((std::string_view) s).substr(n + 1), ","));
+}
+
+StorePathWithOutputs Store::parsePathWithOutputs(const std::string & s)
+{
+    auto [path, outputs] = nix::parsePathWithOutputs(s);
+    return {parseStorePath(path), std::move(outputs)};
+}
+
 }