Tweaks to make Cabal play nicer with haddock
The path for the html docs now includes the package name at the end, which works nicer for multiple packages sharing a contents/index. Use --ghc-pkg when available (in haddock darcs only currently) to tell haddock which ghc-pkg to use. Use --allow-missing-html when available (in haddock darcs only currently) to tell haddock not to worry if it can't find the HTML for packages we depend on. This is necessary when haddocking a group of packages before moving them all into place.