refactor: isolate Engine from Expert

[doorgan]

The burrito packaging will require the Engine app to be fully isolated from Expert, this PR does that

• In order to build the Engine app with one elixir version and have Expert load it for the project node, Expert must not depend on Engine(other than for tests)
• A lot of the code in Engine was actually being setup and used by Expert, so a lot of code had to be moved either to Expert, or to Forge
• Both Expert and Engine used shared data structures like %Diagnostics{} that were defined in the Engine application. Those datastructures were moved to Forge to remove the dependency of Expert on Engine
• We need to perform two namespacing tasks for two different contexts: once for the Engine app so it can be packaged, and once for Expert so the shared data structures get namespaced. Otherwise Expert would try to use Forge.Project and Engine would try to use XPForge.Project, causing crashes.