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.