[POC] Show progress when resolving dependencies

Hi, this is just an illustrative PR for discussion.

When installing dependencies on large-ish projects I can sometimes be waiting for 10 minutes before composer starts to install packages, and the output of composer doesn't indicate any progress. 

I think it would be beneficial to output messages before time-consuming processes begin. These messages could be at the `-v` level.

I would propose passing either the `OutputInterface` to the `Solver` class, or a logging / profiling service which would access the Output by proxy.