Update to Cabal head, update ghc-pkg to use new module re-export types

The main change is that Cabal changed the representation of module
re-exports to distinguish reexports in source .cabal files versus
re-exports in installed package registraion files.

Cabal now also does the resolution of re-exports to specific installed
packages itself, so ghc-pkg no longer has to do this. This is a cleaner
design overall because re-export resolution can fail so it is better to
do it during package configuration rather than package registration.
It also simplifies the re-export representation that ghc-pkg has to use.
Update to Cabal head, update ghc-pkg to use new module re-export types

Summary:
The main change is that Cabal changed the representation of module
re-exports to distinguish reexports in source .cabal files versus
re-exports in installed package registraion files.

Cabal now also does the resolution of re-exports to specific installed
packages itself, so ghc-pkg no longer has to do this. This is a cleaner
design overall because re-export resolution can fail so it is better to
do it during package configuration rather than package registration.
It also simplifies the re-export representation that ghc-pkg has to use.

Add extra ghc-pkg sanity check for module re-exports and duplicates

For re-exports, check that the defining package exists and that it
exposes the defining module (or for self-rexport exposed or hidden
modules). Also check that the defining package is actually a direct
or indirect dependency of the package doing the re-exporting.

Also add a check for duplicate modules in a package, including
re-exported modules.

Test Plan:
So far the sanity checks are totally untested. Should add some test
case to make sure the sanity checks do catch things correctly, and
don't ban legal things.

Reviewers: ezyang, austin

Subscribers: simonmar, ezyang, carter

Differential Revision: https://phabricator.haskell.org/D183