Add mapMG to allow making ModuleGraph abstract

Currently GHC exposes the internal details of `ModuleGraph`
which inhibits making `ModuleGraph` support faster lookups.

Haddock relies on the internal representation by using `map`
on `ModuleGraph`. See also
https://github.com/haskell/haddock/issues/635

Adding `mapMG` should allow us to make `ModuleGraph` abstract.

Test Plan: ./validate

Reviewers: simonmar, austin, bgamari, alexbiehl

Reviewed By: bgamari, alexbiehl

Subscribers: alexbiehl, rwbarton, thomie

Differential Revision: https://phabricator.haskell.org/D3645