formula_auditor: disallow external tap dependencies in core

[carlocab]

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Occasionally, we get PRs that attempt to add a dependency on a formula
in an external tap in homebrew/core (e.g. Homebrew/homebrew-core#94791).

We don't actually have an audit that disallows that. We should have one.

[BrewTestBot]

Review period will end on 2022-02-10 at 23:00:13 UTC.

[BrewTestBot]

Review period skipped due to critical label.