This is a stopgap fix for GHC 8.0 bug #12485: in particular,
it relaxes need for -package-db flags to be given in
dependency order.  The trade-off is that we are a lot more
unsafe when there are packages with duplicate 'id's in
the database stack: the new code will not do an ABI compatibility
check: if two packages have the same 'id', they are assumed to
be ABI compatible.  If this is not true, GHC may build
segfaulting executables.

Missing test updates, but I'm putting it up so people can
take a look.

In GHC 8.2, we'll record ABIs for all dependencies, allowing
GHC to make better decisions about shadowing.

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

Test Plan: validate

Reviewers: austin, niteria, bgamari, simonmar

Reviewed By: simonmar

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2613

GHC Trac Issues: #12485