Correctly account for -package-db ordering when picking packages.
Summary: When I originally implemented ABI-based shadowing as per ee4e1654, I switched our strategy from pasting together lists to creating a map of all units first, and then selecting packages from this. However, what I did not realize when doing this was that we actually depended on the *ordering* of these lists later, when we selected a preferred package to use. The crux is if I have -package-db db1 -package-db db2 -package p-0.1, and p-0.1 is provided by both db1 and db2, which one does the -package flag select? Previously, this was undetermined; now we always select the instance from the LATEST package database. (If p-0.1 shows up multiple times in the same database, once again the chosen package is undefined.) The reason why cabal08 intermittently failed was that, in practice, we were sorting on the UnitId, so when we bumped version numbers, that often wibbled the UnitIds so that they compared oppositely. I've extended the test so that we check that the relation is antisymmetric. Fixes #13313 Signed-off-by: Edward Z. Yang <email@example.com> Test Plan: validate Reviewers: bgamari, austin Subscribers: rwbarton, thomie Differential Revision: https://phabricator.haskell.org/D3369