Support for generating Debianization from Cabal data.
Expand the contents of the .substvars file for a library package. Each cabal package corresponds to a directory name-version, either in usrlib or in usrlibhaskell-packagesghc/lib. In that directory is a compiler subdirectory such as ghc-6.8.2. In the ghc subdirectory is one or two library files of the form libHSname-version.a and libHSname-version_p.a. We can determine the debian package names by running dpkg -S on these names, or examining the varlibdpkginfo/*.list files. From these we can determine the source package name, and from that the documentation package name.