Prevents overwrite of linked dependencies

When a linked package has a dependency that itself is linked, the link was replaced with the installed dependency package. This update causes the update command, and by extension, the link command, to not overwrite linked dependencies of a linked package.

This can be replicated by creating three bower packages and issuing the following commands:

/src/A/bower.json:
```json
{
  "name": "A",
  "version": "0.0.0",
  "license": "MIT",
  "private": true,
  "dependencies": {
    "B": "/src/B"
  }
}
```

/src/B/bower.json:
```json
{
  "name": "B",
  "version": "0.0.0",
  "license": "MIT",
  "private": true,
  "dependencies": {
    "C": "/src/C"
  }
}
```

/src/C/bower.json:
```json
{
  "name": "C",
  "version": "0.0.0",
  "license": "MIT",
  "private": true
}
```

Then, create a link to C in A:
```shell
cd /src/C
bower link
cd /src/A
bower link C
```


Finally, create a link to B in A:
```shell
cd /src/B
bower link
cd /src/A
bower link B
```

We expect that linking B into A should not remove the link that A has on C.