Convert dasource and gen-packages from svn to git

* Convert gen-packages to use the git repository, which for now involves
  hard-coding paths and using Github.  It can really only be used by
  someone on debathena-root at this point.
* gen-packages now writes a packages file that includes the paths (on
  git.mit.edu) to the repositories
* Convert dasource to use git, and correctly checkout and clean package
  build directories.  We can no longer determine by a pathname whether
  daconfiscate is needed, so we use our best guess, because we're
  phasing that out anyway.
* dasource will look for old svn checkouts in package build directories
  and kill them if needed
Add sanity check in dasource

Ensure that if dasource is invoked with no args, the invoker really
meant to build source packages for ALL THE PACKAGES EVER
Ensure the packages file is named correctly

The file generated should be named packages, not packages.git.
The latter was used in internal testing only.