formula: reject dot dirs as racks

[ilovezfs]

• 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 tests with your changes locally?

---

Dot directories are not racks because formulae names do not start with
"." so avoid attempting to descend into dot directories, which may not
always be possible (e.g. .Trashes).

Fixes #3700.

[alyssais]

This is getting kind of long — we might want to split this out into multiple chained #reject calls for readability. LGTM either way though!