tap_auditor: check formulae names in autobump.txt

[cho-m]

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes? Here's an example.
• Have you successfully run brew style with your changes locally?
• Have you successfully run brew typecheck with your changes locally?
• Have you successfully run brew tests with your changes locally?

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We audit all other formulae lists and this allows identifying when a removed formulae has leftover/outdated references.

In future, may want to refactor autobump handling to give it its own DSL so that we can explicitly label certain formulae as non-autobumped for a specific reason (e.g. upstream autobumps, consistently broken for last bump attempts, etc.)

[p-linnane]

Good idea on the DSL. Would love to see that.

[Bo98]

I'm now OK with a DSL now that we have brew bump --auto. It didn't make sense before when it was tied to a random homebrew-core workflow but if we unhide the --auto flag and support it for third-party taps then the DSL makes sense.

[carlocab]

Alternative suggestion to a DSL: #18227 (comment)

[cho-m]

Probably can discuss further either in dedicated issue or whenever anyone gets chance for PR.