formula: make extract_macho_slice_from private

[carlocab]

• 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?

---

Based on feedback from #11756.

[BrewTestBot]

Review period will end on 2021-08-19 at 03:53:14 UTC.

[BrewTestBot]

Review period ended.