Alec Theriault authored Dec 27, 2018 and Ben Gamari committed Jan 16, 2019

This means that we can query the package DB for haddock interfaces.
Haddock uses this in its testsuite. 'cabal {v1-,v2,}-haddock' also
uses this.

Also thread through to Haddock package-specific overrides of GHC
warnings.