Generate real (but empty) object files for signatures.

Summary:
It's not great, but it preserves a nice invariant that every Haskell
source file has an object file (we already have a hack in place ensure
this is the case for hs-boot files) and further ensures every package
has a library associated with it (which would not be the case if
the package had all signatures and we didn't make object files.)

Contains Cabal submodule update.

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

Test Plan: validate

Reviewers: simonpj, austin

Subscribers: carter, thomie

Differential Revision: https://phabricator.haskell.org/D548