The JB hotfix has a weird history, that means it was never really supposed to see such a wide audience, which means it's not deployed as carefully as the other packages.
The TL;DR being that it's built from the 5.4.x JB tree, so when I said the main JB package, I meant that one (the one tagged "K5 JB" in the snapshots listing).
The actual source tree is public, but it's an old SVN repo with a complex layout, so you're better off looking at the actual package, as it's complete and already laid out properly, unlike the repo
. This should be self-evident once you take an actual look at the build scripts, because they're all there, and they're responsible for piecing it all together.
(You might be encountering links to the Trac instance instead of the repo. The Trac instance is down because Python 3 shenanigans and lack of time to deal with it. The SVN repo is up, though).