Refactor build pipeline to support baremetal and containerized builds (C++ and Python APIs)

[sa1g]

Hi
I'm currently working on using UCCL in a baremetal environment with the C++ API, and I'd like to propose some improvements to the build pipeline that could benefit the project more broadly.

Specifically, I think it would be valuable to introduce explicit build targets for:

• C++ only
• Python API

This would help avoid linking unused dependencies (e.g., gtest, gflags) in the compiled library and make the build process more transparent and modular. Also, possibly, adding a simple way to build for debug or release.

I've started prototyping an xmake build script, but since it's still in the early stages and I'd eventually like to see it merged into main, I wanted to gather feedback before proceeding further. Some potential benefits of this approach include:

• Removing submodules and managing dependencies directly through xmake, simplifying the repository structure
• Build rules for release and debug
• Targets for both C++ only and with python wheels
• Separating source code from tests and benchmarks for a cleaner organization (though I'm not sure if this would be desirable)
• If needed build.sh could be adapted to run with xmake, thus making the build process transparent to users

Alternatively, if a full xmake migration is too invasive, I could focus on cleaning up the current build pipeline by:

• Adding baremetal and no-python flags to build.sh
• Using these flags to skip containerized builds and avoid building Python wheels when not needed
• Removing unused dependencies from the build (Makefiles ofc)

this requires a lot less effort and would virtually do most of the tasks as the one above and would require way less testing, but it isn't as clean

I'd appreciate any thoughts or preferences on which direction would be most helpful for the project. Happy to iterate based on feedback!

Thanks!