Bugfix #3724

[AvrahamO]

Summary

Fix #3724

When running yarn install with a customized --modules-folder, binaries are still linked under the default node_modules/.bin.

This PR fixes that behavior.

From now on:

1. install and add commands will link binaries under custom/.bin.
2. run command will prepend the custom folder path to PATH env.
3. remove command will remove the specified package from the custom folder. The same way install --force will prune extraneous from custom folder.

Test plan

All the following were tested with local .yarnrc too.

Add

$ yarn --modules-folder custom add acorn
$ tree -a --noreport custom/.bin/
custom/.bin/
└── acorn -> ../acorn/bin/acorn

Install

$ cat package.json 
{
  "dependencies": {
    "acorn": "^5.7.1"
  }
}
$ yarn --modules-folder custom install
$ tree -a --noreport custom/.bin/
custom/.bin/
└── acorn -> ../acorn/bin/acorn

Run

$ cat package.json 
{
  "scripts": {
    "start": "acorn --help"
  },
  "dependencies": {
    "acorn": "^5.7.1"
  }
}
$ yarn --modules-folder custom start
$ acorn --help
usage: acorn [--ecma3|--ecma5|--ecma6|--ecma7|--ecma8|--ecma9|...|--ecma2015|--ecma2016|--ecma2017|--ecma2018|...]
        [--tokenize] [--locations] [---allow-hash-bang] [--compact] [--silent] [--module] [--help] [--] [infile]
Done in 0.26s.

Remove

$ yarn remove acorn
$ tree -a --noreport custom
custom
└── .yarn-integrity

[arcanis]

Looks good, thanks 👍