[Proposal] [WIP] Customizable publish paths

Related: #4013

Currently, the various publish commands assume in a very rigid fashion that config files reside within "/src/config", view files inside "/src/views" and so on (relative to the package root). This is not very flexible, especially with PSR-4 autoloading becoming more popular.

Here's one idea on how to handle it - keep an array of registered package paths on the Application class, let service providers add to that array and then somehow pass those on to every Publisher and/or PublishCommand class.

Thoughts?