- This builds on @jonchang's work that started in #6265.
- We now use `uses_from_macos` to declare dependencies that are implicit
  on macOS because they ship with macOS, but they're needed on Linux. We
  have to be sure that the dependencies people specify as
  `uses_from_macos` are actually shipped with macOS. So, we maintain a
  safelist of those dependencies and check against it.
- Also add more legitimate `uses_from_macos` dependencies to the list.
- This is runnable with `brew audit --only-cops=FormulaAudit/UsesFromMacos`.
- It produces different number of failures on macOS vs. Linux, because
  apparently we've not synced Homebrew/linuxbrew-core upstream thoroughly
  enough yet.
- Originally this was designed as a `--strict` audit, but we flipped it
  to be a normal audit because - to quote Mike - this is "sufficiently
  robust" now.