Lerna bootstrap does not link local dependencies

<!--- Provide a general summary of the issue in the Title above -->

I have configured two modules in a mono repo to use lerna with a local dependency. I expected that



```bash

$ lerna bootstrap

$ lerna run test

```



would be sufficient to download all external dependencies, link a local dependency and execute and pass all tests in all modules.



## Expected Behavior

As per the [lerna bootstrap](https://github.com/lerna/lerna/tree/master/commands/bootstrap#usage) documentation:



> 2. Symlink together all Lerna packages that are dependencies of each other.



Thus, I expected that `lerna bootstrap` would create a symlink in `module-b/node_modules` below that points to `module-a` (which then would allow the tests to be executed and pass).



## Current Behavior



No linking occurs, which cause the tests to fail:



> lerna ERR! yarn run test exited 1 in 'module-b'

> lerna ERR! yarn run test stdout:

> yarn run v1.19.1

> $ jest

> info Visit https://yarnpkg.com/en/docs/cli/run for documentation about this command.

> 

> lerna ERR! yarn run test stderr:

> FAIL ./import.test.js

>   ● Test suite failed to run

> 

>     Cannot find module 'module-a' from 'import.test.js'

> 

>     > 1 | const moduleA = require('module-a');

>         | ^

>       2 | 

>       3 | test('should import module-a', () => {

>       4 |   moduleA();

> 

>       at Resolver.resolveModule (node_modules/jest-resolve/build/index.js:259:17)

>       at Object.<anonymous> (import.test.js:1:1)

> 





## Steps to Reproduce

1. Create the folder structure below

2. Copy the files inlined to their corresponding directory

3. Execute `$ lerna bootstrap` followed by `$ lerna run test`



```

project-root

   + packages

   |       + module-a

   |       |      + package.json

   |       |      + index.js

   |       |

   |       + module-b

   |       |      + package.json

   |       |      + import.test.js

   |

   +  lerna.json     

```





<details><summary>packages/module-a/package.json</summary><p>

<!-- browsers demand the next line be empty -->



```json

{

  "name": "module-a",

  "version": "1.0.0",

  "private": true,

  "main": "index.js",

  "scripts": {

    "test": "echo \"Test passed in module-a\""

  }

}

```

</p></details>

<details><summary>packages/module-a/index.js</summary><p>

<!-- browsers demand the next line be empty -->



```javascript

module.exports = () => console.log('Log in module-a');

```

</p></details>





<details><summary>packages/module-b/package.json</summary><p>

<!-- browsers demand the next line be empty -->



```json

{

  "name": "module-b",

  "version": "1.0.0",

  "private": true,

  "main": "index.js",

  "scripts": {

    "test": "jest"

  },

  "dependencies": {

    "module-a": "file:../module-a"

  },

  "devDependencies": {

    "jest": "^24.9.0"

  }

}

```

</p></details>

<details><summary>packages/module-b/import.test.js</summary><p>

<!-- browsers demand the next line be empty -->



```javascr

[]