Add index on derivation_outputs id and derivation_id fields

This might help with doing recursive queries on the derivations graph.

sqitch/verify/derivation_outputs_id_and_derivation_id_idx.sql

@@ -0,0 +1,7 @@
+-- Verify guix-data-service:derivation_outputs_id_and_derivation_id_idx on pg
+
+BEGIN;
+
+-- XXX Add verifications here.
+
+ROLLBACK;