This commit is a bit bigger than it could have: Meson changes could have gone in separately from CI and Debian.

This commit looks more complicated than it should reasonably be. Alas, Cargo is a piece of work, and it doesn't let honest people just choose different versions of dependencies, leading to a cascade of misery. Several things were tried to curb the disaster:

- Cargo [feature] supports choosing dependencies, but doesn't support specifying dependency versions
- Cargo has a cfg() syntax in sections for choosing dependencies by build options, but it explicitly doesn't support selecting on features…
- Cargo allows choosing different dependencies based on features, so perhaps dependencies with different versions could live in stub crates pulled in as needed? Nope! If a dependency doesn't exist in the repo (and that's the point here), Cargo throws up its hands.

This means Cargo.toml needs to be generated based on the build type. More misery:

- we lose th...