handle empty packages

[nponeccop]

Few packages are just umbrellas and don't have any .hs files. Cabal builds them just fine but cblrepo chokes on haddock.

A notable example is diagrams meta-package.

[magthe]

I've run into this only once before and then it was easily handled by patching. From your description above it sounds like it wouldn't for diagrams though.

It would require a bit more inspection of the .cabal to find out if the lib/exe has no source files.