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/deploy/package_versions_by_guix_revision_range.sql

@@ -0,0 +1,14 @@
+-- Deploy guix-data-service:package_versions_by_guix_revision_range to pg
+
+BEGIN;
+
+CREATE TABLE package_versions_by_guix_revision_range (
+  git_repository_id integer NOT NULL REFERENCES git_repositories (id),
+  branch_name varchar NOT NULL,
+  package_name varchar NOT NULL,
+  package_version varchar NOT NULL,
+  first_guix_revision_id integer NOT NULL REFERENCES guix_revisions (id),
+  last_guix_revision_id integer NOT NULL REFERENCES guix_revisions (id)
+);
+
+COMMIT;