In order to mitigate syntax highlighting issues and a rust-analyzer bug
(https://github.com/rust-analyzer/rust-analyzer/issues/10178), rename
files that are included with the `include!` macro to have a `.rs`
extension.
Make sure the included files are outside `src/`, in order to not confuse
humans and automated editing tools that might mistake them for valid
Rust modules.
fixes arti#381