"Third-party" is a Googleism that doesn't make much sense otherwise.
Most references to crates just say "crate", implying that they are
open-source packages available on https://crates.io, so this updates a
few additional locations to do the same.
The files in this directory make up the content of the course. The files here
can include third-party content from ../third_party/ as well.
When we publish a translation of the course, we git restore the src/ and
third_party/ directories at the repository root back to the date listed in the
POT-Creation-Date header of the translation. It is crucial, that all
translatable content lives in those two directories. The other files (such as
book.toml and theme/) are not restored and we always use the latest version
of them.