[Distutils] Keep downloaded files in case of an error

[Robin Bryce]

python setup.py develop --build-directory=build worked for me::

robin at rsb2-ubuntu:~/devel/setuptools-test$ python setup.py develop -b build
running develop
running egg_info
writing requirements to lib/foopackage.egg-info/requires.txt
writing lib/foopackage.egg-info/PKG-INFO
writing top-level names to lib/foopackage.egg-info/top_level.txt
writing entry points to lib/foopackage.egg-info/entry_points.txt
reading manifest file 'lib/foopackage.egg-info/SOURCES.txt'
writing manifest file 'lib/foopackage.egg-info/SOURCES.txt'
running build_ext
Creating /home/robin/devel/0root/lib/python2.4/site-packages/foopackage.egg-link
(link to lib)
foopackage 0.0.0 is already the active version in easy-install.pth
Installing fooconf script to /home/robin/devel/0root/bin

Installed /home/robin/devel/setuptools-test/lib
Processing dependencies for foopackage==0.0.0
Searching for Twisted>=2.4.0
Reading http://www.python.org/pypi/Twisted/
Reading http://twistedmatrix.com/
Reading http://www.python.org/pypi/Twisted/2.4.0
Best match: Twisted 2.4.0
Downloading http://tmrc.mit.edu/mirror/twisted/Twisted/2.4/Twisted-2.4.0.tar.bz2Processing
Twisted-2.4.0.tar.bz2
Running setup.py -q bdist_egg --dist-dir build/twisted/egg-dist-tmp-n9yA5h
usage: setup.py [global_opts] cmd1 [cmd1_opts] [cmd2 [cmd2_opts] ...]
   or: setup.py --help [cmd1 cmd2 ...]
   or: setup.py --help-commands
   or: setup.py cmd --help

error: invalid command 'bdist_egg'
Error: Subprocess exited with result 1 for project core
error: Setup script exited with 1

robin at rsb2-ubuntu:~/devel/setuptools-test$ ls build/twisted
LICENSE   TwistedConch-0.7.0  TwistedNames-0.3.0   TwistedWords-0.4.0
README    TwistedCore-2.4.0   TwistedNews-0.2.0    ZopeInterface-3.1.0c1
setup.py  TwistedLore-0.2.0   TwistedRunner-0.2.0
temp      TwistedMail-0.3.0   TwistedWeb-0.6.0


and::

robin at rsb2-ubuntu:~/devel/setuptools-test$ python setup.py develop --help

<snip>

  --build-directory (-b)         download/extract/build in DIR; keep the
                                 results