Add an index on package_derivations_by_guix_revision_range

This helps render the package version range related pages.

sqitch/verify/package_range_index.sql

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