Renamespacing

[eggrobin]

The piles of usings throughout the codebase are getting unwieldy, unmaintainable, and uninformative. Consider ephemeris_body.hpp: https://github.com/mockingbirdnest/Principia/blob/b0bec96cae90105aafa39131f140449fd995936d/physics/ephemeris_body.hpp#L35-L69 What use is it to know that we use Quotient and not Product? Not everything in that list is actually used anyway: J2000 is unused, and merely reflects implementation details from 6 years ago.

These usings also interact poorly with modules: in a modularized world, either we have to put them in the global namespace, which makes them even more verbose (using principia::quantities::Length rather than using quantities::Length), or have to put them between the opening of the namespace and the beginning of an export block (meaning we cannot use export namespace).

There also can sometimes be an ambiguity between these usings that exist for convenience and those that are part of the API, like these in cgs.hpp: https://github.com/mockingbirdnest/Principia/blob/b0bec96cae90105aafa39131f140449fd995936d/quantities/cgs.hpp#L18-L19

We want to switch to using namespace for those convenience usings, with smaller namespaces.

Current	This issue	Modularized future
#include "other_project_name/other_file_name.hpp" // […] #include "yet_another_project_name/cætera.hpp" namespace principia { namespace project_name { namespace internal_file_name { using other_project_name::Something; using other_project_name::SomethingElse; // […] using yet_another_project_name::Cætera; // […] } // namespace internal_file_name using internal_file_name::ExportedEntity; // […] } // namespace project_name } // namespace principia	#include "other_project_name/other_file_name.hpp" // […] #include "yet_another_project_name/cætera.hpp" namespace principia::project_name::file_name { namespace internal { using namespace principia::other_project_name::other_file_name; // […] using namespace principia::yet_another_project_name::cætera; // […] } // namespace internal using internal::ExportedEntity; // […] } // namespace principia::project_name::file_name	export module principia.project_name.file_name import principia.other_project.other_file_name; // […] import principia.yet_another_project_name.cætera; using namespace principia::other_project_name::other_file_name; // […] using namespace principia::yet_another_project_name::cætera; export namespace principia::project_name::file_name { // […] } // namespace principia::project_name::file_name