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/deploy/package_derivations_by_guix_revision_range.sql

@@ -0,0 +1,17 @@
+-- Deploy guix-data-service:package_derivations_by_guix_revision_range to pg
+
+BEGIN;
+
+CREATE TABLE package_derivations_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,
+  derivation_id integer NOT NULL,
+  system varchar NOT NULL,
+  target 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;

sqitch/revert/package_derivations_by_guix_revision_range.sql

@@ -0,0 +1,7 @@
+-- Revert guix-data-service:package_derivations_by_guix_revision_range from pg
+
+BEGIN;
+
+DROP TABLE package_derivations_by_guix_revision_range;
+
+COMMIT;

sqitch/sqitch.plan

@@ -25,3 +25,4 @@ index_on_load_new_guix_revision_job_events_job_id 2019-09-29T10:39:04Z Christoph
 fix_null_values_in_git_branches 2019-09-29T11:06:12Z Christopher Baines <mail@cbaines.net> # Fix the NULL values in git_branches
 add_retry_value_to_job_event_enum 2019-10-02T19:13:52Z Christopher Baines <mail@cbaines.net> # Add retry value to job_event enum
 remove_guix_revision_duplicates 2019-10-05T08:00:14Z Christopher Baines <mail@cbaines.net> # Remove duplicates in the guix_revisions table
+package_derivations_by_guix_revision_range 2019-11-09T19:09:48Z Christopher Baines <mail@cbaines.net> # Add package_derivations_by_guix_revision_range

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;