How to clear out the info downloaded via apt-get update?

For testing purposes, to ensure that the automation I am running will work on a "fresh" server, how can I clear all info downloaded via apt-get update?

apt-get clean does not appear to do this.

Example sequence:

1. Check apt show somepackage and get no results.
2. Add a new sources list in /etc/apt/sources.list.d
3. Repeat step 1 and get no results.
4. Run apt-get update
5. Repeat step 1 and the package is now shown.
6. ???? Run some command to reverse the effects of step 4.
7. Repeat step 1 and get no results.

I even removed the new sources list and did an apt-get clean and still got a result, listed as an "additional record" and only shown in full with -a.

(Forgive me for not including the actual package details, but I am working against an in-house repository so the results would not be reproducible anyway.)

What is the most complete way to clear out cached information to accomplish step 6 above?