Summary:
This commit reimplements shadowing on package databases by doing
the shadowing calculation on a per-database basis: specifically,
if a later package database shadows a package from the earlier
databases, we first remove that package (and its transitive
dependencies) before merging the databases together.

This should also fix bootstrapping GHC HEAD with HEAD.

Signed-off-by: Edward Z. Yang <ezyang@cs.stanford.edu>

Test Plan: validate

Reviewers: ggreif, bgamari, austin

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D1385