This makes the output deterministic.  This is important to me as I am
using `Packages` index files as a kind of lockfile and committing it
to my git repository.  Without this we get a lot of noise in the diff
whenever the file is regenerated because
[go randomises map iteration order][1].

[1]: https://nathanleclaire.com/blog/2014/04/27/a-surprising-feature-of-golang-that-colored-me-impressed/

My measly contribution hardly merits it but it's a requirement in
`CONTRIBUTING.md`.