When a third_party library includes mongo/ headers, it constrains our ability to improve those headers for use in our main codebase.

While investigating SERVER-41495 (upgrade libfmt), we discovered a recent problem with libfmt (https://github.com/fmtlib/fmt/issues/1197), in which its header causes malfunctions in the presence of a using namespace std. The s2geometry library internally uses such directives, so it can't include the fmt/format.h header. Because s2geometry has been locally modified to #include "mongo/logger/log_severity.h", it now brings in all the headers mongo/logging brings in, which now include fmt/format.h.

So s2geometry fails to build due to an unrelated libfmt upgrade.
We should isolate third_party code so that it doesn't directly include mongo's full headers for systems like logging. If we need to inject entities into a third_party library we have to find a way to do it through an abstraction so they don't depend on our headers directly. We can't control what third_party code will do the way we can control our own code, so we have to keep the third_party libraries separated from each other.