Hadrian: configure packages with right 'htmldir'
This means that we can query the package DB for haddock interfaces and
count on haddockInterfaces
and haddockHTMLs
being valid paths.
Haddock uses this in its testsuite as does cabal {v1-,v2,}-haddock
.
Also threads through to Haddock package-specific overrides of GHC warnings.