Rework cross derivations support

Stop using the system values as targets, and remove package derivation entries
where this is the case.

Switch the non-cross derivation case to have a target of "", rather than
matching the system, as this makes more sense, and is more consistent now that
the target values no longer match the system values.

Hardcode some more correct target values, and use these instead. Hopefully
this can be better integrated with Guix in the future.

This commit also includes a migration attempting to shrink some indexes.

sqitch/verify/change_package_derivations_target.sql

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