WIP: Link command

First of all, this pullrequest is a **work in progress**.

I'm determined to finish this, but I've got stuck for three days now. I have no clue how to force the composer to use the package, that has <code>installation-source: link</code> that I've provided. The DependencyResolver is just a gibberish to me.

If you guys could just push me in the right direction I'd be really happy to finish this. #601
I think this part if you were not required if, you have deleted is good.