Implement PoC to allow the config to configure the load precedence.

[pelson]

(NOTE: PR to my own fork, for discussion, but not a proposal for merging to pip yet)

Prototype of config load order configuration.

With this change, you can find the load precedence when debugging:

$ python -m pip config debug
env-var:
env:
global:
  /etc/xdg/xdg-ubuntu/pip/pip.conf, exists: False
  /etc/xdg/pip/pip.conf, exists: False
  /etc/pip.conf, exists: False
site:
  /media/important/github/pypa/pip/env/pip.conf, exists: True
user:
  /home/pelson/.pip/pip.conf, exists: False
  /home/pelson/.config/pip/pip.conf, exists: False
load-precedence: global, user, site, env, env-var

You can also influence it: