Move StorePathWithOutputs into its own header/file

In the following commits it will become less prevalent.

src/libstore/path-with-outputs.cc

@@ -0,0 +1,36 @@
+#include "store-api.hh"
+
+namespace nix {
+
+std::string StorePathWithOutputs::to_string(const Store & store) const
+{
+    return outputs.empty()
+        ? store.printStorePath(path)
+        : store.printStorePath(path) + "!" + concatStringsSep(",", outputs);
+}
+
+
+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)};
+}
+
+
+StorePathWithOutputs Store::followLinksToStorePathWithOutputs(std::string_view path) const
+{
+    auto [path2, outputs] = nix::parsePathWithOutputs(path);
+    return StorePathWithOutputs { followLinksToStorePath(path2), std::move(outputs) };
+}
+
+}