that PackageIndex handles all downloading of any kind, Installers can be
reused for multiple packages, and main() manages temporary directories and
all communication between PackageIndex and Installer.  Also, change
run_setup to take an argument sequence, because later we will probably need
other arguments to control other aspects of run_setup's behavior.

--HG--
branch : setuptools
extra : convert_revision : svn%3A6015fed2-1504-0410-9fe1-9d1591cc4771/sandbox/trunk/setuptools%4041050