Create a second Store::getFSAccessor for a single store object

This is sometimes easier / more performant to implement, and
independently it is also a more convenient interface for many callers.

The existing store-wide `getFSAccessor` is only used for

 - `nix why-depends`
 - the evaluator

I hope we can get rid of it for those, too, and then we have the option
of getting rid of the store-wide method.

Co-authored-by: Sergei Zimmerman <sergei@zimmerman.foo>

src/libstore/dummy-store.cc

@@ -276,7 +276,14 @@ struct DummyStore : virtual Store
         callback(nullptr);
     }
 
-    virtual ref<SourceAccessor> getFSAccessor(bool requireValidPath) override
+    std::shared_ptr<SourceAccessor> getFSAccessor(const StorePath & path, bool requireValidPath) override
+    {
+        std::shared_ptr<SourceAccessor> res;
+        contents.cvisit(path, [&](const auto & kv) { res = kv.second.contents.get_ptr(); });
+        return res;
+    }
+
+    ref<SourceAccessor> getFSAccessor(bool requireValidPath) override
     {
         return wholeStoreView;
     }