Arch: Mark deleted packages as dead and hide them from listings

We've got a lot of packages in the DB that have long been removed from
the Arch repos. These are still indexed, but won't clutter the package
listing anymore.

Also fixed an issue with packages.id numbers getting rather large
because the indexer allocates a new ID for every package on every
update.

sql/update-2021-12-13b.sql

@@ -0,0 +1,3 @@
+ALTER TABLE packages
+    ALTER COLUMN category SET NOT NULL,
+    ADD COLUMN dead     boolean   NOT NULL DEFAULT FALSE;