Support registry entries that must match exactly

An example use is for pinning the "nixpkgs" entry the system-wide
registry to a particular store path. Inexact matches
(e.g. "nixpkgs/master") should still use the global registry.

src/libfetchers/registry.hh

@@ -23,6 +23,7 @@ struct Registry
         std::shared_ptr<const Input> from;
         std::shared_ptr<const Input> to;
         Attrs extraAttrs;
+        bool exact = false;
     };
 
     std::vector<Entry> entries;