Distribute stdlib types as types-stdlib

This has been briefly discussed before, but I think it would be a good idea if we started distributing the stdlib types in a separate package types-stdlib, similar to third-party packages:

• Type checkers could optionally depend on (a specific version of) that package, instead of shipping their own copy, reducing coupling and arguably improving maintainability.
• Third-party stubs could require a minimum version of the types-stdlib package, for example if they are relying on _typeshed features or for cases like #5762. Type checkers that opt to distribute a copy of the stdlib, could warn if incompatible stubs are used.

Technical considerations:

• The versioning scheme would be similar to third-party stubs, e.g. types-stdlib 3.10.X, where X is incremented automatically. We should probably increment the version number as soon as the first beta is released and we are reasonably sure we have added all new/changed API.
• The stubs would be installed into a special directory, for example stdlib, or even a non-importable name like types-stdlib (so we'd have stubs stdlib/sys.pyi etc.) so that they are only used by type checkers that support this package and don't interfere.

首先审阅 issue 讨论以及之前链接的上下文，因为其中没有指定文件、测试或入口点。在实现之前，确定包布局、版本方案、安装行为和兼容性预期；完成的要求是就设计达成一致，并为 stdlib stubs 提供完整的分发路径。