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

What is the current behavior?
Incompatibility with npm 5.

consider the following package.json (reduced for brevity)

{
  "name": "my-project",
  "description": "my project",
  "main": "index.js",
  "dependencies": {
      "my_local_lib": "file:LocalLib"
   }
}

with npm 5 the lib "my_local_lib" is installed as a symlink
but yarn install it as copy of the folder LocalLib

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

1. create a new project
2. create a sub project (folder named FolderX with a package.json)
3. yarn add FolderX
4. yarn install

What is the expected behavior?
create a symlink of FolderX

Please mention your node.js, yarn and operating system version.
node v8.4.0
yarn v0.27.5
macOS 10.12.6