I just successfully installed the project with the following settings:

[tool.uv.build-backend]
module-name = "a_library.a_module"
namespace = true

I’ve been aware of this namespace option the whole time, but I’d been avoiding it because the documentation scared me with this warning:

Using namespace = true disables safety checks. Using an explicit list of module names is strongly recommended outside of legacy projects.

But now, I’m fine with using this option. Sorry for bothering you.