Support for abi-depends for computing shadowing.

Summary:
This is a complete fix based off of
ed7af26606b3a605a4511065ca1a43b1c0f3b51d for handling
shadowing and out-of-order -package-db flags simultaneously.

The general strategy is we first put all databases together,
overriding packages as necessary.  Once this is done, we successfully
prune out broken packages, including packages which depend on a package
whose ABI differs from the ABI we need.

Our check gracefully degrades in the absence of abi-depends, as
we only check deps which are recorded in abi-depends.

Contains time and Cabal submodule update.

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

Test Plan: validate

Reviewers: niteria, austin, bgamari

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2846

GHC Trac Issues: #12485