[multibody] `model_directives.h` (only) uses namespace `multibody::parsing`

[RussTedrake]

model_directives.h, process_model_directives.h, and scoped_names.h stand out as the only headers in multibody/parsing that use the namespace multibody::parsing. All other headers/classes in this directory simply use the namespace multibody. This causes small but unnecessary friction when having to remember/check the namespace for the various parsing methods.

Expected: these files should use namespace multibody only, like the other headers/classes in this directory.

cc @ggould-tri as the original author

[jwnimmer-tri]

I would buy the main entry points LoadModelDirectives and ProcessModelDirectives moving into the simpler namespace, but I think many of the helper types in other files (e.g., AddWeld or PrefixName) are named too generically to live in multibody as-is. For those names, the clarity that they are the parsing flavor of the name is important. We could rename them as part of the move (so that they could stand on their own), but I'm not sure it's worth the effort. I assume the major point of friction is only with the entry point?

[RussTedrake]

Yes. Moving only the entry points would resolve my concern.

[jwnimmer-tri]

Now that dmd.yaml can go in through the Parser API directly, my bid would be to investigate deprecating process_model_directives.h, instead of changing its namespace.

That's probably too aggressive, so perhaps more subtly: try to figure out why people are still using process_model_directives.h, and help them port to something better.