includePaths doesn't recognise node_modules in yarn workspace

I've got an app in yarn workspace. When I'm trying to use bootstrap-sass, when I specify
```
includePaths: [
'node_modules/bootstrap-sass/assets/stylesheets',
],
```
build fails with error `Directory not found: node_modules/bootstrap-sass`

If I use `../node_modules/bootstrap-sass/assets/stylesheets` instead, it works.

This is happening because in yarn workspace bootstrap-sass is installed on the top level of yarn workspace.

All other ember-cli stuff resolves `node_modules` in yarn workspace correctly.