Move all of pip's CLI handling into `pip._internal.cli`

• Allows us to break up pip._internal.__init__
• Provides a clearer place to put all of the autocompletion support code.
• Better grouping of the code that does one thing

Suggested rearrangement (I’ll omit pip._internal and say pip instead):

before	after
pip.__init__.autocomplete()	pip.cli.autocompletion.autocomplete()
pip.basecommand	pip.cli.basecommand
pip.baseparser	pip.cli.option_parser
pip.cmdoptions	pip.cli.option_templates
pip.commands	pip.cli.commands

[pfmoore]

@techalchemy Could you please at least add a huge disclaimer to the docs that makes it clear that the pip maintainers take no responsibility for any breakage as a result of using this package?

[techalchemy]

(Giant warning added btw)