Add a new table to store package versions by revision ranges

This isn't new information, but derived from information already in the
database. It's collected here to make querying faster.

The table is updated when each new revision is entered.

sqitch/verify/package_versions_by_guix_revision_range.sql

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