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/derivations.hh

@@ -56,14 +56,10 @@ struct BasicDerivation
        the given derivation. */
     Path findOutput(const string & id) const;
 
-    bool willBuildLocally() const;
-
     bool substitutesAllowed() const;
 
     bool isBuiltin() const;
 
-    bool canBuildLocally() const;
-
     /* Return true iff this is a fixed-output derivation. */
     bool isFixedOutput() const;