Auto merge of rust-lang#105018 - zertosh:path_buf_deref_mut, r=dtolnay

Implement DerefMut for PathBuf

Without this, there's no way to get a `&mut Path` from `PathBuf` without
going through `into_boxed_path`. This is relevant now that rust-lang#105002 adds
`PathBuf::as_mut_os_string` and `Path::as_mut_os_str`.

Author: bors