Make clear that StorePathWithOutputs is a deprecated type

- Add a comment

- Put `OutputsSpec` in a different header (First part of #6815)

- Make a few stray uses of it in new code use `DerivedPath` instead.

src/libstore/outputs-spec.cc

@@ -0,0 +1,61 @@
+#include "outputs-spec.hh"
+#include "nlohmann/json.hpp"
+
+#include <regex>
+
+namespace nix {
+
+std::pair<std::string, OutputsSpec> parseOutputsSpec(const std::string & s)
+{
+    static std::regex regex(R"((.*)\^((\*)|([a-z]+(,[a-z]+)*)))");
+
+    std::smatch match;
+    if (!std::regex_match(s, match, regex))
+        return {s, DefaultOutputs()};
+
+    if (match[3].matched)
+        return {match[1], AllOutputs()};
+
+    return {match[1], tokenizeString<OutputNames>(match[4].str(), ",")};
+}
+
+std::string printOutputsSpec(const OutputsSpec & outputsSpec)
+{
+    if (std::get_if<DefaultOutputs>(&outputsSpec))
+        return "";
+
+    if (std::get_if<AllOutputs>(&outputsSpec))
+        return "^*";
+
+    if (auto outputNames = std::get_if<OutputNames>(&outputsSpec))
+        return "^" + concatStringsSep(",", *outputNames);
+
+    assert(false);
+}
+
+void to_json(nlohmann::json & json, const OutputsSpec & outputsSpec)
+{
+    if (std::get_if<DefaultOutputs>(&outputsSpec))
+        json = nullptr;
+
+    else if (std::get_if<AllOutputs>(&outputsSpec))
+        json = std::vector<std::string>({"*"});
+
+    else if (auto outputNames = std::get_if<OutputNames>(&outputsSpec))
+        json = *outputNames;
+}
+
+void from_json(const nlohmann::json & json, OutputsSpec & outputsSpec)
+{
+    if (json.is_null())
+        outputsSpec = DefaultOutputs();
+    else {
+        auto names = json.get<OutputNames>();
+        if (names == OutputNames({"*"}))
+            outputsSpec = AllOutputs();
+        else
+            outputsSpec = names;
+    }
+}
+
+}