tap_constants: allow formulae to have @ in name.

Follow-up from #812 to fix handling fully-qualified versioned formulae
names.

Allows pulling https://github.com/Homebrew/homebrew-core/pull/971.

Library/Homebrew/tap_constants.rb

 # match taps' formulae, e.g. someuser/sometap/someformula
-HOMEBREW_TAP_FORMULA_REGEX = %r{^([\w-]+)/([\w-]+)/([\w+-.]+)$}
+HOMEBREW_TAP_FORMULA_REGEX = %r{^([\w-]+)/([\w-]+)/([\w+-.@]+)$}
 # match taps' directory paths, e.g. HOMEBREW_LIBRARY/Taps/someuser/sometap
 HOMEBREW_TAP_DIR_REGEX = %r{#{Regexp.escape(HOMEBREW_LIBRARY.to_s)}/Taps/([\w-]+)/([\w-]+)}
 # match taps' formula paths, e.g. HOMEBREW_LIBRARY/Taps/someuser/sometap/someformula