tap parser ignores keg_only

the tap formula parser never reads keg_only, so every tap formula is treated as not keg-only. this was hidden by the @ name check in is_keg_only, which #403 removes.

once that lands, versioned tap formulas that declare keg_only :versioned_formula get linked. e.g. shivammathur/php/php@8.1 would fail with a link conflict if another php is already linked.

fix: parse keg_only and the reason in parse_tap_formula_ruby.