extend/os/mac: install svn on catalina and newer

[dawidd6]

Subversion is no longer available on Catalina and supposedly won't be available in newer macOS releases.

With this PR we can avoid specifying svn as build dependency, like here:
Homebrew/homebrew-core#59776

Failed workflow run (only on Catalina):
https://github.com/Homebrew/homebrew-core/runs/993661520?check_suite_focus=true#step:5:44

• 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?

---

[Bo98]

Does Utils.svn_available? work?

If so, do we still need a macOS-specific override for subversion_dep_if_needed?

[dawidd6]

➜  local/Homebrew master ✓  brew config
HOMEBREW_VERSION: 2.4.9-88-g8d97029
ORIGIN: https://github.com/Homebrew/brew
HEAD: 8d97029b03d4054d7923c55827d28b6d4c3a351c
Last commit: 3 weeks ago
Core tap ORIGIN: https://github.com/Homebrew/homebrew-core
Core tap HEAD: e7fb51895ffcb9a0c5e4a90f2f9230acd0e4fa45
Core tap last commit: 3 weeks ago
HOMEBREW_PREFIX: /usr/local
HOMEBREW_BAT: set
HOMEBREW_BINTRAY_KEY: set
HOMEBREW_BINTRAY_USER: dawidd6
HOMEBREW_DEVELOPER: set
HOMEBREW_EDITOR: nvim
HOMEBREW_GITHUB_API_TOKEN: set
HOMEBREW_MAKE_JOBS: 4
HOMEBREW_NO_AUTO_UPDATE: set
HOMEBREW_NO_EMOJI: set
CPU: quad-core 64-bit haswell
Homebrew Ruby: 2.6.3 => /System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/bin/ruby
Clang: 12.0 build 1200
Git: 2.24.3 => /Library/Developer/CommandLineTools/usr/bin/git
Curl: 7.64.1 => /usr/bin/curl
macOS: 10.15.6-x86_64
CLT: 12.0.26.2
Xcode: N/A
➜  local/Homebrew master ✓  brew ruby -e "p Utils.svn_available?"
false

Also:

➜  homebrew/homebrew-core ✓  svn
svn: error: The subversion command line tools are no longer provided by Xcode.
➜  homebrew/homebrew-core ✓  which svn
/usr/bin/svn

[Bo98]

Sounds like the generic one here would work fine then:

brew/Library/Homebrew/dependency_collector.rb

Lines 62 to 66 in 42477a1

	def subversion_dep_if_needed(tags)
	return if Utils.svn_available?
	Dependency.new("subversion", tags)
	end

unless there's a reason it wasn't done before.