Including `.inc` files baffles rust-analyzer

When rust-analyzer sees us do include!(FILENAME), where FILENAME doesn't end with .rs, it complains with "failed to load file."

This appears to be an instance of https://github.com/rust-analyzer/rust-analyzer/issues/10178

We use include! in two places:

• tor-proto/src/crypto/cell.rs, where we include a testdata file full of encrypted cells.
• tor-dirmgr/src/config.rs, where we include a set of fallback directories.

Our options seem to be:

• wait for the rust-analyzer bug to get fixed
• Tolerate spurious errors from rust-analyzer
• Rename these files to end with .rs, even though they aren't valid rust modules.
• Find some other clever workaround

Options I wouldn't like:

• Including these files verbatim in the main source code files. (The test vectors are ugly, and the fallback directories are machine-generated.)