audit: add "homebrew/core" for relevant checks

[jonchang]

• 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 tests with your changes locally?

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This should help contributors and maintainers of third-party taps to more clearly see which audits are specific to Homebrew-core. This will also be a prelude to including more core checks by default.

[MikeMcQuaid]

Good idea. What do you think about using homebrew-core, Homebrew/core or Homebrew/homebrew-core instead?

[jonchang]

Good idea. What do you think about using homebrew-core, Homebrew/core or Homebrew/homebrew-core instead?

I considered this (mainly Homebrew/core or Homebrew-core as it is closest to our usual style) but audits tend to be quite terse and economial. I'm open to changing this though.

[MikeMcQuaid]

Another option would be core tap. I'm fine with this being merged as-is, though, up to you!

[jonchang]

Here are the current references to "core" in audits and custom rubocops:

dev-cmd/audit.rb:353:problem "Formula name conflicts with existing core formula."
dev-cmd/audit.rb:954:problem "`env :std` in `core` formulae is deprecated" if line.include?("env :std")
rubocops/lines.rb:204:problem "Don't use #{check.source}; Homebrew/core only supports macOS"
rubocops/urls.rb:238:"Homebrew/homebrew-core is source-only."

[jonchang]

I've changed this to homebrew/core throughout in line with our prose guidelines. Since I consider audits to be user-facing this should conform to the guidelines and be as clear as possible without introducing new jargon to new contributors.

[jonchang]

Tests are green; merging. Thanks for the quick review @MikeMcQuaid!