Fix @nativescript/core modules imports

[mukaschultze]

PR Checklist

• [ ] The PR title follows our guidelines: https://github.com/NativeScript/NativeScript/blob/master/CONTRIBUTING.md#commit-messages.
• [ ] There is an issue for the bug/feature this PR is for. To avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it.
• [ ] All existing tests are passing
• [ ] Tests for the changes are included

What is the current behavior?

What is the new behavior?

Closes #16, #17.