Fix #15594 (--abi-hash with Backpack sometimes fails)

Authored by ezyang on Nov 11 2018, 9:39 PM.

Description

Fix Trac #15594 (--abi-hash with Backpack sometimes fails)

Summary:
For holes, its necessary to "see through" the instantiation
of the hole to get accurate family instance dependencies.
For example, if B imports <A>, and <A> is instantiated with
F, we must grab and include all of the dep_finsts from
F to have an accurate transitive dep_finsts list.

However, we MUST NOT do this for regular modules.
First, for efficiency reasons, doing this
bloats the the dep_finsts list, because we *already* had
those modules in the list (it wasn't a hole module, after
all). But there's a second, more important correctness
consideration: we perform module renaming when running
--abi-hash. In this case, GHC's contract to the user is that
it will NOT go and read out interfaces of any dependencies
(https://github.com/haskell/cabal/issues/3633); the point of
--abi-hash is just to get a hash of the on-disk interfaces
for this *specific* package. If we go off and tug on the
interface for /everything/ in dep_finsts, we're gonna have a
bad time. (It's safe to do do this for hole modules, though,
because the hmap for --abi-hash is always trivial, so the
interface we request is local. Though, maybe we ought
not to do it in this case either...)

Signed-off-by: Edward Z. Yang <ezyang@fb.com>

Test Plan: validate

Reviewers: alexbiehl, goldfire, bgamari

Subscribers: ppk, shlevy, rwbarton, carter

GHC Trac Issues: Trac #15594

Differential Revision: https://phabricator.haskell.org/D5123