Fix #1063530: FTBFS with nodejs 18.19.0

The first commit is the actual fix; the commit that actually moves index.d.ts's content to undici-types was left behind, so 18.19.0's @types/node was unable to import it, resulting in all node packages that import @types/node breaking. Unfortunately, that commit doesn't actually build, because 18.19.0 is already in -security, and undici does need @types/node, so the second commit breaks the cycle by forcing @types/node to import the local copy of undici-types. (There may be cleaner ways of solving this? but this is the simplest way that I'm aware of.)

Aside: I'm not sure if there's a canonical way of citing Debian bugs from commit messages? If there is, I'll happily add the bug URL to them.