stop enabling dependencies in build.rs by default?

(I'm basically using issues as a communication mechanism here)

See https://github.com/coreos/cargo-vendor-filterer/issues/71

TL;DR when rustix 0.37 came along with https://github.com/bytecodealliance/rustix/commit/a2c6c7a8b7adcf46592b7519ba90ecece18b4bd6 this ended up enabling `libc` by default and that pulled in the `errno` dependency by default.

However, `cargo metadata` doesn't see this dependency, it's only enabled via `build.rs`.

And that ends up breaking `cargo vendor-filterer`.

Since in reality, libc is enabled by default, ISTM we can rework the features and build scripts here right?

首先检查 build.rs 以及启用 libc 和 errno 的 feature 定义，然后将 cargo metadata 报告的依赖关系图与 cargo vendor-filterer 的行为进行比较。当默认依赖配置在不依赖 build.rs 单独激活的情况下得到一致表示，并且 cargo vendor-filterer 不再失败时，该问题即得到解决。

由索引模型根据 Issue 内容生成。