Move structured attrs handling into a separate class

This is primarily because Derivation::{can,will}BuildLocally() depends
on attributes like preferLocalBuild and requiredSystemFeatures, but it
can't handle them properly because it doesn't have access to the
structured attributes.

src/libstore/parsed-derivations.hh

@@ -0,0 +1,33 @@
+#include "derivations.hh"
+
+#include <nlohmann/json.hpp>
+
+namespace nix {
+
+class ParsedDerivation
+{
+    Path drvPath;
+    BasicDerivation & drv;
+    std::experimental::optional<nlohmann::json> structuredAttrs;
+
+public:
+
+    ParsedDerivation(const Path & drvPath, BasicDerivation & drv);
+
+    const std::experimental::optional<nlohmann::json> & getStructuredAttrs() const
+    {
+        return structuredAttrs;
+    }
+
+    std::experimental::optional<std::string> getStringAttr(const std::string & name) const;
+
+    bool getBoolAttr(const std::string & name, bool def = false) const;
+
+    std::experimental::optional<Strings> getStringsAttr(const std::string & name) const;
+
+    bool canBuildLocally() const;
+
+    bool willBuildLocally() const;
+};
+
+}