setup haddock fails for packages without modules

[bos]

(Imported from Trac #954, reported by @nomeata on 2012-05-20)

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.

Bumped into this recently; caused a spurious build failure of a codebase I maintain.

[ddssff]

Same here.

[OlivierSohn]

Same here, for a legitimate use case (a convenience library building just c++ sources)