[wip] Improve ability to detect Python 2 during build

[smashwilson]

Description of the Change

Use @atom/dowsing-rod to aggressively detect python 2 installations and pass it to node-gyp subprocesses. We won't be able to get away from requiring python 2 entirely, but we can at least accept a python 2 path from npm config get python or ${PYTHON}.

Without this in place, even with ${PYTHON} set to a python2 install, the build for git-utils fails with a python 2/3 compatibility problem. It turns out that the Makefile generated by node-gyp sometimes executes a file with a #!/usr/bin/env python shebang line, so unless the python2 binary is actually discoverable on the ${PATH} as "python", the build fails.

Alternate Designs

The clearest alternative is to actually fix node-gyp at the source, by porting gyp to Python 3 or JavaScript. That would require getting a patch accepted into gyp, then waiting for it to be included in a node-gyp release, then a node release, then upgrade our build to use that node version.

Why Should This Be In Core?

It's where the build is.

I'm also going to create a parallel pull request for apm to improve our Python detection during apm builds (atom/apm#775).

Benefits

Developers who:

• Use homebrew on MacOS and have the python formula installed.
• Use Arch Linux or other distributions that use Python 3 as their default python package.

...will be able to build Atom without having to break things on their system that rely on Python 3 being the default. We'll also have a nice message if Python 2 cannot be found.

Possible Drawbacks

Verification Process

The ultimate test is to run script/build with Python 3 on the PATH.

Applicable Issues

• Incorrect Python version used on Arch Linux git-utils#48
• Installation fails at node-gyp rebuild on OSX 10.10 git-utils#72
• Python2 regression with 1.15.3 apm#698
• Some modules fail to build under windows apm#706
• Respect a user's PYTHON or npm_config_python settings apm#703

[Arcanemagus]

Re: Alternate designs: nodejs/node-gyp#1337

[smashwilson]

There's also this: nodejs/node-gyp#960

[smashwilson]

I believe (hope?) that this has been obsoleted by new npm/node-gyp releases.