hackage-build puts unwanted files in docs.tar

[komadori]

As pointed out in issue #119, when the hackage-build program builds a new package and it has dependencies, all the haddock output is placed into the same directory tree and used to generate docs.tar. This causes docs.tar to contain documentation files for those dependencies in addition to the documentation files for the target package. These additional files are not referenced or useful on the Hackage server as inter-package links correctly to point each package's own docs directory. They simply bloat the size of the docs.tar.

[dcoutts]

Lets get this fixed, should be easy. Volunteers please!