Allow generating RBIs in Rails engines

[vinistock]

In a Rails engine, the Rails application is typically defined inside test/dummy. However, it's a real application and we could load it to inspect the defined models and controllers.

Currently, it does not work because we return early if config/application.rb is not defined in the current directory.

If we could somehow allow this to also point to different paths, like test/dummy/config/application.rb, DSL generation for engines would just work.