setup haddock fails for packages without modules

(Imported from [Trac #954](http://hackage.haskell.org/trac/hackage/ticket/954), reported by @nomeata on 2012-05-20)

[http://hackage.haskell.org/package/diagrams-0.5](http://hackage.haskell.org/package/diagrams-0.5) is a meta-package, i.e. only depends on other packages. Running setup haddock fails:

$ ./Setup haddock --hyperlink-source

Running Haddock for diagrams-0.5...

Running hscolour for diagrams-0.5...

Preprocessing library diagrams-0.5...

Warning: The documentation for the following packages are not installed. No

links will be generated to these packages: rts-1.0

Preprocessing library diagrams-0.5...

haddock: No input file(s).

Not a big deal, but still a corner-case that could be fixed.