npm link scoped package bug fix #5758

Just need to check pkg is scoped or not.

```bash
$ ~/git/npm/bin/npm-cli.js link @ali/tnpm-publish-test
/git/ab/node_modules/@ali/tnpm-publish-test -> /git/nvm/v0.11.12/lib/node_modules/@ali/tnpm-publish-test -> /git/tnpm-publish-test
```