Fallback to yarn when npm isn't installed

• ESLint Version: 4.0.0
• Node Version: v7.10.0
• npm Version: None! Due to some issues with npm, I’m testing a yarn-only development flow.

What did you do? Please include the actual source code causing the issue.

yarn add --dev eslint
./node_modules/.bin/eslint --init

What did you expect to happen? It would be great if eslint didn’t rely on npm to be installed, and ran yarn if npm was not found.

What actually happened? Please include the actual, raw output from ESLint.

$ node_modules/.bin/eslint --init
? How would you like to configure ESLint? Use a popular style guide
? Which style guide do you want to follow? Airbnb
? Do you use React? No
? What format do you want your config file to be in? JavaScript
Installing eslint-plugin-import, eslint-config-airbnb-base
/bin/sh: 1: npm: not found
Command failed: npm i --save-dev eslint-plugin-import eslint-config-airbnb-base
Error: Command failed: npm i --save-dev eslint-plugin-import eslint-config-airbnb-base
    at checkExecSyncError (child_process.js:481:13)
    at Object.execSync (child_process.js:521:13)
    at Object.installSyncSaveDev (/z/prg/js/node_modules/eslint/lib/util/npm-util.js:56:18)
    at installModules (/z/prg/js/node_modules/eslint/lib/config/config-initializer.js:99:17)
    at getConfigForStyleGuide (/z/prg/js/node_modules/eslint/lib/config/config-initializer.js:277:5)
    at inquirer.prompt.then.earlyAnswers (/z/prg/js/node_modules/eslint/lib/config/config-initializer.js:357:22)

[not-an-aardvark]

I think it might be easiest to just assume that npm is present, and print a useful error message if it’s not. Given that npm is bundled with node, I suspect it’s quite rare for users to not have npm installed. So we could do everything else, and then tell the user in an error message that “to finish the setup, you need to install these packages somehow.”

[not-an-aardvark]

Fixed in https://github.com/eslint/eslint/pull/9169.