Allow for including pip packages in source tar

[slaclau]

Packages build with dh_virtualenv cannot be uploaded to Launchpad ppas as they require network access to pypi at build time. I have worked around this by downloading wheels before building my source and passing —no-index and —find-links to pip. Unfortunately this causes pip install . to fail. I get around this by building a wheel before building the source and passing —skip-install, followed by pip install pkgname.whl.The shebangs in any executables then need fixing as they have absolute paths in them from my build machine.