fix: show all diffs either way

[beckermr]

Checklist

• [x] Used a static YAML file for the patch if possible (instructions).
• [x] Only wrote code directly into generate_patch_json.py if absolutely necessary.
• [x] Ran pre-commit run -a and ensured all files pass the linting checks.
• [x] Ran python show_diff.py and posted the output as part of the PR.
• [x] Modifications won't affect packages built in the future.

This PR fixes a bug where diffs that involve a package not in the current index do not show. It is a weird edge case that we basically never encounter in practice except when I did.

[conda-forge-webservices[bot]]

Hi! This is the friendly automated conda-forge-linting service.

I just wanted to let you know that I linted all conda-recipes in your PR (recipe/meta.yaml) and found it was in an excellent condition.

[beckermr]

pre-commit.ci autofix

[github-actions[bot]]

Hi! This is the friendly conda-forge automerge bot!

I considered the following status checks when analyzing this PR:

• linter: passed
• azure: passed

Thus the PR was passing and merged! Have a great day!