Yarn add with workspaces bug showcase

See yarnpkg/yarn#4513

Steps I followed:

1. Create an empty directory(workspace-1), and add a minimal package.json file.
2. In root package.json, set "workspaces": [ "./*" ]
3. Run:

cd workspace-1
yarn add lodash.assign

4. By now, I'd have (also see screenshot)

/workspace-1
  /node_modules
    /lodash.assign

While I would have expected

/node_modules
  /lodash.assign
/workspace-1

With node modules being installed at the root level.

5. Now running yarn at the root directory installs the lodash.assign dependency at the root.

System info and versions:

$ uname -srvo
Linux 4.4.0-93-generic #116-Ubuntu SMP Fri Aug 11 21:17:51 UTC 2017 GNU/Linux

$ lsb_release -rcd
Description:    Ubuntu 16.04.3 LTS
Release:        16.04
Codename:       xenial

$ node --version
v8.5.0

$ npm --version
5.3.0

$ yarn --version
1.0.2-20170921.1145