Rewrite `sysconfig.get_config_vars` data generation to receive clearly-defined inputs

Feature or enhancement

Proposal:

Right now, some of the data is calculated during import, then we use that to calculate more data when building the sysconfig.get_config_vars cache. It would be good to transform all of this into a clearly defined algorithm, where we the data is generated based on a set of inputs, making it easier to generate data when cross-compiling, and allowing us to generate data for a specific environment (eg. source builds, vs installed build). This would have helped mitigate issues such as GH-127429, GH-128055, and GH-126789.

Additionally, this also gives us the opportunity to split the data into two sets: the minimum needed to evaluate the install scheme paths, and the non-critical additional data we expose in sysconfig.get_config_vars. This would help with the bootstrapping issue in python -m sysconfig --generate-posix-vars.

https://github.com/python/cpython/blob/f802c8bf872ab882d3056675acc79c950fe5b93c/Lib/sysconfig/__main__.py#L206-L221

Has this already been discussed elsewhere?

No response given

Links to previous discussion of this feature:

No response

从 Lib/sysconfig/main.py 开始，重点查看链接的第 206–221 行以及 python -m sysconfig --generate-posix-vars 流程。跟踪配置数据在导入和缓存生成期间是如何计算的。完成的标准是：数据生成算法具有明确定义的输入，支持交叉编译和已安装构建等不同环境，并将安装方案数据与额外公开的变量分离。