haddock-project: support for sublibraries

This commit makes haddock-project handle sublibraries.

The commit changes how `cabal haddock` works, changing the layout
in the `dist-newstyle` folder.  With this change `haddock` subcommand
will install `package:sublib` component in a directory `package-sublib`
under `l/sublib/doc/html/`.