Investigate improving internals with Composer Installer #48

I had previously discussed with Jordi about using the Composer Installer system to manage download/build/install process. I'd done some previous investigation work into this in a now outdated branch (https://github.com/asgrim/pie/tree/experiment-with-composer-installer) but at the time, it felt a bit of a rabbit hole. Now that we've got PIE approximately working, it would be worth revisiting this approach and take advantage of the Composer internals more.