Add an index on package_derivations_by_guix_revision_range

This helps render the package version range related pages.

sqitch/deploy/package_range_index.sql

@@ -0,0 +1,7 @@
+-- Deploy guix-data-service:package_range_index to pg
+
+BEGIN;
+
+CREATE INDEX range_package_name_idx ON package_derivations_by_guix_revision_range (package_name);
+
+COMMIT;