utils/shared_audits: GitHub urls can have '.' in repo and '/' in tag

[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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Also clean up logic using String slices

dot example: z.lua
https:/Homebrew/homebrew-core/blob/master/Formula/z/z.lua.rb#L4

slash example: hz
https:/Homebrew/homebrew-core/blob/master/Formula/h/hz.rb#L4