easy_install: wrap pip

[ghost]

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one thing to keep in mind is that module mode I don't think works on py26, meaning that pip will not be used for that version.

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@jaraco Before I proceed, is this a valid approach?

[jaraco]

This PR relates to #917.

This is interesting. The first thing I notice is there are several stylistic changes here. While I'm not opposed to stylistic changes, they do distract from the meaningful changes. May I suggest resisting the temptation to make stylistic changes, or if they're important to the implementation, make them as a separate pull request first so they're not visible during the review of the functional changes?

I think you're on the right track here, and I'm eager to see where you take this. I'm a bit concerned about the inclusion of a dependency on pip, as it starts to create a weird circular dependency where pip requires setuptools to build packages during install, but setuptools uses pip to install packages during builds.

Other challenges that need to be considered:

• Should pip install affect all easy_install or just setup_requires/tests_require? (the former will affect more
• What are the use cases expected by the current implementation? Do you intend to replace them in a compatible way or will it be backward-incompatible? If the latter, what are the implications (and actions necessary for a system upgrading)?
• Although the test suite is meant to reflect the required use cases, it's not 100%, so wherever possible, consider ensuring the presence of tests to validate the behavior as it is now (or how we want it to be).

I'll be honest, while I'm hopeful that pip might replace easy_install, it may prove a difficult transition to make. I welcome the effort, but with the understanding that it may prove infeasible to roll out. Anything you can do to help with that effort (soliciting feedback on distutils-sig) will help a lot in getting this ultimately accepted.

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I'm a bit concerned about the inclusion of a dependency on pip, as it starts to create a weird circular dependency where pip requires setuptools to build packages during install, but setuptools uses pip to install packages during builds.

I was concerned about this but I remember that other people assured me it would not be a problem.

I think you're on the right track here, and I'm eager to see where you take this.

Actually given that egg and whl are both zip formats, it would be possible to use the wheel command and then add those to the pythonpath.