Move `builder` configuration to the top-level to enable reuse

[stackotter]

Builders should be defined at the top level (next to apps and projects) so that they can be reused multiple times within the same project. This will allow people to include builders from other projects as well by specifying a git URL for the project and the name of the builder to use. Their source path should also be separate to the name.

This should happen before 3.0 because it's a breaking change. Well, at least the configuration format change should happen before then.