Move the build loop to a service

[caendesilva]

I like keeping console commands light, as I think they should mainly be an easy way to access and trigger services. Think of it as the frontend to an API. Currently the main Hyde static site building process is done in the HydeBuildStaticSiteCommand, and while I managed to put a lot of the logic into traits I do still think the command class can be simplified by moving to a service, which would also use the new router.

The reason I've had everything in the command is because I did not want to lose the Symfony progress bars, but I'm rather sure that we can access those by passing the output interface to the service.