Consider bundling code into a package (again)

We moved away from storing our code here as a package for ergonomic reasons: [targets doesn't play nice with pkgload::load_all()](https://github.com/ropensci/targets/discussions/1211). This meant that every time you wanted to run a pipeline, you had to renv::install() the package, which significantly slowed down development.

There are drawbacks to not packaging this code:
- the main one I'm feeling now is lack of a docs site and a vignettes page
- lack of a package NAMESPACE (we just load everything) and dependencies managed purely by renv (arguably not so bad)
- some awkwardness around loading code and calling tests (also minor)

I think I'm still happy with the trade we've made so far, but once the discussion in the thread above resolves, it should unblock this issue.