extend/pathname: limit `write` override to a refinement

Library/Homebrew/build.rb

Library/Homebrew/dev-cmd/extract.rb

Library/Homebrew/extend/pathname.rb

Library/Homebrew/formula_creator.rb

Library/Homebrew/formulary.rb

Library/Homebrew/test/completions_spec.rb

Library/Homebrew/test/dev-cmd/audit_spec.rb

Library/Homebrew/test/formulary_spec.rb

Library/Homebrew/test/pathname_spec.rb

Library/Homebrew/test/tab_spec.rb

Library/Homebrew/test/tap_spec.rb