Start storing and identifying sets of derivation outputs

Derivations are effectively equivalent if they produce the same set of
outputs, which is possible because of the equivalence of fixed output
derivations. A fixed output derivation can be different, but equivalent,
because it produces the same fixed output.

To better allow tracking equivalent derivations, primarily to allow working
out what derivations might correspond to a build, store the sets of derivation
outputs, and which derivations they relate to.

sqitch/verify/derivation_output_sets.sql

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