[ghc-pkg] shut up about warnings at -v0

ghc-pkg loves to be chatty, and is very happy to tell you that your haddock-html: and haddock-interfaces: are broken.

Just try something like:

$ ghc init db
$ ghc-pkg -f my/package.conf.d describe vector | ghc-pkg -v0 --force --global-package-db db register -

and even at -v0 it will keep telling you about your broken haddock-html and haddock-interfaces.

This makes ghc-pkg shut up at -v0.