Factored the "sub-command" machinery out to Command.  Mainly, this meant removing 'get_sub_commands()', and moving the 'sub_commands' class attribute to the end and restructuring it to conform to the new regime.
Factored the "sub-command" machinery out to Command.  Mainly, this
meant removing 'get_sub_commands()', and moving the 'sub_commands' class
attribute to the end and restructuring it to conform to the new regime.

pypa/setuptools,pypa/setuptools,pypa/setuptools