Builder can fail to upload docs with long filenames

[gbaz]

Tar.pack is partial on long filenames.

Cf: https://hackage.haskell.org/package/tar-0.5.0.3/docs/Codec-Archive-Tar.html#g:7

On a package such as http://hackage.haskell.org/package/stratosphere-0.3.0 this can lead, apparently, to a failure in the builder. Not sure of the right solution...

@dcoutts @bgamari thoughts?

[gbaz]

The interesting thing is that the tarfile for the package itself is I think ok -- its just the generated documentation ends up with a few more characters in the strings and it pushes things over the limit :-/

Maybe we should just change the builder to be resilient to this type of error?

[gbaz]

one other fix would be to extend the tar library to use the posix pax interchange specs as per: haskell/tar#1