Enforce finalized sorbet methods

[dduugg]

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We have a few methods marked :final:

$ git grep 'sig(:final)'
Library/Homebrew/downloadable.rb:  sig(:final) { returns(T::Boolean) }
Library/Homebrew/formula.rb:  sig(:final) {
Library/Homebrew/requirement.rb:  sig(:final) {
Library/Homebrew/requirement.rb:  sig(:final) { returns(T::Boolean) }
Library/Homebrew/requirement.rb:  sig(:final) {

However, per sorbet docs, enforcement requires an incantation of T::Configuration.enable_final_checks_on_hooks setup.

I've made that change here, added a test that a specific finalized method can't be overriden, and done some other minor sorbet tuning.

[MikeMcQuaid]

Thanks @dduugg!