Tomorrow's plan: teach it to install a binary package, based on the imported pkg_summary input.

I'll do this in multiple steps:

1. Download the appropriate tarball into the store. Since I have the SHA512 hash of it, I can use that as the tarball's hash ahead of time.
2. Create a .drv that "builds" an output from the tarball.
3. Actually implement the builder (a shell script will do for now).
4. Teach it to install dependencies too.

Hm, come to think of it. I could get rid of a partition if I treated sets as binary packages too, and shoved them into the store.