This allows additional commands to be provided for existing setup.py

This allows additional commands to be provided for existing setup.py
scripts without modifying either the distutils installation or the
setup.py scripts of packages with which the new commands will be used.

Specifically, an option is added to distutils that allows additional
packages to be searched for command implementations in addition to
distutils.command.  The additional packages can be specified on the
command line or via the installation or personal configuration files
already loaded by distutils.

For discussion, see the thread starting with:

http://mail.python.org/pipermail/distutils-sig/2004-August/004112.html

This closes SF patch #102241.