This uses explicitly enumerated packages= and package_data= arguments to
setup(), rather than relying upon the convenient (but darcs-specific)
functions which would determine these values by asking the revision-control
system.
Note that darcsver is still used, when building from a darcs tree.
MANIFEST.in is a script where you express all the files that you want to be
included (on top of the ones that setuptools figured out for itself).
setuptools_darcs_plugin is a setuptools plugin that makes all files which are
under darcs revision control get included.
