Use BuildableReq for buildPaths and ensurePath

This avoids an ambiguity where the `StorePathWithOutputs { drvPath, {}
}` could mean "build `brvPath`" or "substitute `drvPath`" depending on
context.

It also brings the internals closer in line to the new CLI, by
generalizing the `Buildable` type is used there and makes that
distinction already.

In doing so, relegate `StorePathWithOutputs` to being a type just for
backwards compatibility (CLI and RPC).

src/libstore/path-with-outputs.hh

@@ -1,6 +1,9 @@
 #pragma once
 
+#include <variant>
+
 #include "path.hh"
+#include "buildable.hh"
 
 namespace nix {
 
@@ -10,8 +13,14 @@ struct StorePathWithOutputs
     std::set<std::string> outputs;
 
     std::string to_string(const Store & store) const;
+
+    BuildableReq toBuildableReq() const;
+
+    static std::variant<StorePathWithOutputs, StorePath> tryFromBuildableReq(const BuildableReq &);
 };
 
+std::vector<BuildableReq> toBuildableReqs(const std::vector<StorePathWithOutputs>);
+
 std::pair<std::string_view, StringSet> parsePathWithOutputs(std::string_view s);
 
 class Store;