Stop calling pip internal APIs to make laniakea work with pip 10.

[nth10sd]

Currently, using Laniakea with pip 10 throws, but works with pip 9.

From pip 10 release notes:

Move all of pip's APIs into the pip._internal package, properly reflecting the fact that pip does not currently have any public APIs. (#4696, #4700)

This means that from pip 10 onwards, we can no longer call pip.main anymore, nor should we rely on it. Instead, we should put the azure package requirement into requirements.txt.

[posidron]

Meh. Is there any other way? I purposed that cause azure is quite heavy and user shouldn't have to fetch it if he doesn't intend to use the azure module.

[posidron]

The only thought which comes to my mind would be to create separate requirement.txt (i.e amazon.txt, azure.txt) files but that probably could get messy in the root folder.

[jschwartzentruber]

Not sure exactly how to set it up, but in celery you can do eg. pip install celery[sqs] and you get celery plus optional dependencies to support the SQS backend. They use a subdirectory of requirements.txt files and a complicated setup.py script.

[jschwartzentruber]

You could also just forget trying to install it, and print a message to the user that azure is required to use the azure backend.

[posidron]

So that link from above https://github.com/celery/celery/tree/master/requirements looks like what I was thinking of. @nth10sd could you take that over and adapt your fix to that? I would rather have it like that, rather than a print statement followed by a sys.exit. It's frustrating as a user to see such kind of aborts caused by missing dependencies.
I am not sure exactly but IIRC there was an option to include a requirements.txt into another one. In that case there should exist also a all.txt file in the newly created requirements folder.

[nth10sd]

@posidron I'd propose for now to unbreak pip 10 with this first, that requirements format in celery seems like another PR.