Clean up Tap.

[reitermarkus]

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Extracted from #16671.

From the changes in Tap#clear_cache you can actually see we were missing quite a few cached variables, which would be done automatically using #16671.

[MikeMcQuaid]

Again: let's wait a week or so before merging. There have been too many user-affecting regressions from refactors recently.

[MikeMcQuaid]

Makes sense although I do wonder whether the T.must error messages are going to be end-user appropriate?

[github-actions]

This pull request has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs.