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 <firstname.lastname@example.org> Test Plan: validate Reviewers: niteria, austin, bgamari Subscribers: thomie Differential Revision: https://phabricator.haskell.org/D2846 GHC Trac Issues: #12485