Move StorePathWithOutputs into its own header/file

In the following commits it will become less prevalent.

src/libstore/path-with-outputs.hh

@@ -0,0 +1,17 @@
+#pragma once
+
+#include "path.hh"
+
+namespace nix {
+
+struct StorePathWithOutputs
+{
+    StorePath path;
+    std::set<std::string> outputs;
+
+    std::string to_string(const Store & store) const;
+};
+
+std::pair<std::string_view, StringSet> parsePathWithOutputs(std::string_view s);
+
+}