Handle scoped packages when removing version

[Taiters]

Please provide enough information so that others can review your pull request:

Fix for #1178 (Scoped packages being incorrectly handled when removing the version)

When stripping the version from a package string, we first separate the base name and directory. This is done so we can separate the scope (if present) from the package name, before using split to remove the package's version. Once removed, we return the full package with path.join

I've moved this code into a split-version utility, which is now called by link-missing-dependencies and npm-utils.

Test plan (required)

Unit tests have been added for split-version.js to cover the following cases:

• Passing a non-scoped package, both with and without a version
• Passing a scoped package, both with and without a version

[matteofigus]

Thanks!