Make `std::os::unix::ucred` module private
Tracking issue: #42839
Currently, this unstable module exists: [`std::os::unix::ucred`](https://doc.rust-lang.org/stable/std/os/unix/ucred/index.html).
All it does is provide `UCred` (which is also available from `std::os::unix::net`), `impl_*` (which is probably a mishap and should be private) and `peer_cred` (which is undocumented but has a documented counterpart at `std::os::unix::net::UnixStream::peer_cred`).
This PR makes the entire `ucred` module private and moves it into `net`, because that's where it is used.
I hope it's fine to simply remove it without a deprecation phase. Otherwise, I can add back a deprecated reexport module `std::os::unix::ucred`.
`@rustbot` label: -T-libs +T-libs-api
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