Tighten the EntryPoint interface

I've also observed that the issue affects .module and .attr and .load. I'm trying to decide what's the best behavior here. I think it should fail early, at construction time, but right now, the interface is open to mutation of the entry point properties. Probably those should be made final.

Originally posted by @jaraco in #488

Additionally, EntryPoint could probably subclass SimpleNamespace except that EntryPoint currently allows for positional arguments. Let's deprecate that behavior and require keyword arguments.

从 EntryPoint 的定义开始，检查其当前的构造和变更行为。将提议的 final properties、keyword-only construction 和 positional-argument deprecation 作为验收标准；只有在接口行为和 deprecation policy 确定并经过验证后，才算完成。