development_tools: add installed? method. (#455)

Library/Homebrew/cmd/install.rb

Library/Homebrew/cmd/reinstall.rb

Library/Homebrew/cmd/upgrade.rb

Library/Homebrew/compat/macos.rb

Library/Homebrew/extend/ENV/super.rb

Library/Homebrew/extend/os/mac/development_tools.rb

Library/Homebrew/formula_installer.rb

Library/Homebrew/os/mac.rb

Library/Homebrew/test/test_formula_installer.rb

Library/Homebrew/test/test_formula_installer_bottle.rb