Track package replacements

Start at least looking for package replacements, and storing the
details (particularly the derivation). I'm looking at doing this so that build
servers using the Guix Data Service can build these derivations.

sqitch/verify/packages_replacement.sql

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