Allow substituting from different storeDir

Substituters can substitute from one store dir to another with a
little bit of help. The store api just needs to have a CA so it can
recompute the store path based on the new store dir. We can only do
this for fixed output derivations with no references, though.

src/libstore/local-store.hh

@@ -142,7 +142,8 @@ public:
     StorePathSet querySubstitutablePaths(const StorePathSet & paths) override;
 
     void querySubstitutablePathInfos(const StorePathSet & paths,
-        SubstitutablePathInfos & infos) override;
+        SubstitutablePathInfos & infos,
+        std::map<std::string, std::string> pathsCA = {}) override;
 
     void addToStore(const ValidPathInfo & info, Source & source,
         RepairFlag repair, CheckSigsFlag checkSigs,