completions: suggests incorrect `-e` slugs when `-t` is used

Each completion script cleverly suggests exercise slugs when the `-e, --exercise` option is used. However, they always search the current directory, even when the `-t, --track-dir` is used. This means that it can e.g. suggest a slug that doesn't exist in the track directory that configlet is operating on.

I guess I'll call this a "bug". Can we look at the argument of `-t` when completing the argument of `-e`?