Yarn: `yarn bin` does not respect `--modules-folder`

[allenwu1973]

Do you want to request a feature or report a bug?
BUG

What is the current behavior?
yarn bin does not respect modules-folder

If the current behavior is a bug, please provide the steps to reproduce.

$ pwd
/tmp/test
$ yarn add webpack --modules-folder vendor/node_modules/
$ ls vendor/node_modules/.bin/
acorn  atob  errno  json5  miller-rabin  mkdirp  rimraf  sha.js  terser  webpack
$ yarn bin webpack --modules-folder vendor/node_modules/
error Couldn't find a binary named "webpack"
$ yarn bin --modules-folder vendor/node_modules/
/tmp/test/node_modules/.bin

What is the expected behavior?
yarn bin should utilize modules-folder flag and report the correct path

Please mention your node.js, yarn and operating system version.
node v8.11.1
yarn v1.13.0
ubuntu 16.04

[rally25rs]

Could modify #6850 to handle this, since that PR hasn't been accepted yet.