Eliminate unguarded `putStrLn` calls for true -v0 in haddock

[gbaz]

Related ticket from cabal: https://github.com/haskell/cabal/issues/5991

As @harpocrates writes there:

"I don't think --no-warnings will be enough. Even besides that, the Haddock codebase still has a bunch of unguarded putStrLn's for emitting various messages.

Once all that fixed, I think it should be enough to make sure that -v0 in Cabal implies -v0 in Haddock."

[gbaz]

Might be an opportunity for more general cleanup and refactoring of haddock's messages and verbosity flags.