Get rid of special include orders and forward declarations

[eggrobin]

Oddities such as the ones listed below are deeply confusing already, and are obviously module-hostile. There are probably better ways to deal with that.

https://github.com/mockingbirdnest/Principia/blob/b0bec96cae90105aafa39131f140449fd995936d/quantities/generators.hpp#L29-L30 https://github.com/mockingbirdnest/Principia/blob/b0bec96cae90105aafa39131f140449fd995936d/quantities/quantities.hpp#L176-L179

Cross-module forward declarations are impossible, so we should probably try to minimize forward declarations as well.