respect pip configuration at Pipfile creation

[hrbonz]

Respect pip index configuration (pip.conf or environment variable) when
creating a new Pipfile.

I'm really not sure about this implementation but I wanted to start the conversation, I've made some choices that I think have a minimal impact.

I couldn't find a convenient method in the pip project that would just hand me out the best option and respect the configuration precedence so I implemented it by hand.

Change mentioned in #856

[hrbonz]

I feel like I should add some tests here

[techalchemy]

Hey @hrbonz thanks for putting this together. Unfortunately altering which source is added to the Pipfile doesn’t quite respect pip.conf because of the way pip already generates its repository index list (which already should respect pip.conf) and because of the way the piptools resolver works (which isn’t affected by the Pipfile).

[hrbonz]

@techalchemy unfotunately, pip.conf is not respected because of the configuration precedence and the use of sources as indexes on the command line with -i/--index-url or --extra-index-url.

That is the base of the issue I described in #856

[techalchemy]

@hrbonz I see what you're saying, but I'm not actually sure this is the right approach here. It's not clear that we actually want to always fall back to the default indexes in the host pip config, since I'm not sure that provides sufficient portability. I do think we would want to respect environment settings, however, but I think there is some discussion to have here (e.g. in #988 this came up)

We have broader design questions to answer but for now, I would be much more prepared to accept something that prioritizes environment level settings (e.g. PIP_INDEX_URL followed by an iterative list over PIP_EXTRA_INDEX_URL) -- this approach will probably need to take into account verify_ssl from the PIP_TRUSTED_HOST environment variable

[techalchemy]

And before you rework, /cc @nateprewitt would you be willing to approve a PR to use PIP_INDEX_URL from the environment if it exists as the primary pip index (i.e. first entry in [[sources]] in the Pipfile? This seems like something that might impact existing users so I would want a second set of eyes on it

[kennethreitz]

We should honor the environment variable instead.

[kennethreitz]

not today, but thank you

[hrbonz]

There must be something I don't get, why force users to set an environment variable when there is already a configuration for a standard tool that gives you this information? This is, moreover, the tool you use under the hood.
Environment variables are useful in some situations but really not ideal most of the time.