[sugar] [PATCH] Bundlebuilder use manifest, fix_manifest function

[Marco Pesenti Gritti]

Landed with some cleanup and little fixes. Please test it and see if I
broke anything in the process.

I removed is_dir (and the po check which use it) because the return
True was too much of an hack. Happy to consider a better fix for that
as a separate patch.

Thanks!
Marco