Generalize exposed-modules field in installed package database

Summary:
Instead of recording exposed-modules and reexported-modules as seperate
fields in the installed package database, this commit merges them into
a single field (exposed-modules).  The motivation for this change is
in preparation for the inclusion of *signatures* into the installed
package database, which may also be reexported.  Merging the representation
means that we can treat reexports uniformly, no matter if they're a normal
module or a signature.

This commit adds a stub for signatures, but that code isn't wired up to
anything yet.

Contains Cabal submodule update to accommodate these changes.

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

Test Plan: validate

Reviewers: simonpj, duncan, austin

Subscribers: thomie, carter, simonmar

Differential Revision: https://phabricator.haskell.org/D421

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