Standardize usage of namespaces & includes

[iAmMichaelConnor]

E.g. there are several confusing namespaces being used in the base rollup files. Perhaps we could get away with a single namespace (Edit: for base rollup)?
Ideally, the namespace structure (and naming) should match the directory structure (and naming).

[ludamad]

My 2cents is C++ is fine without namespaces at all, and one for a library is polite but I find nested namespaces voluntary fiction that I usually choose not to do. But C++ is notoriously opinionated in practice so I respect the group decision

[dbanks12]

As part of this PR, remove all cases of using-entire-namespace (using namespace <MYNAMESPACE>;) which leads to pollution.

When this PR is done, the following WarningsAsErrors omissions should be removed from .clang-tidy which will promote this deviation from our standard to an error: -google-build-using-namespace, (config here and remove/update comments here and here).