Add a new table to describe the history of derivations

There's already the package_versions_by_guix_revision_range table, but I think
it would be also useful to be able to see how derivations change over time.

sqitch/verify/package_derivations_by_guix_revision_range.sql

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