tap_constants: allow formulae to have @ in name.

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

Allows pulling Homebrew/homebrew-core#971.

Library/Homebrew/tap_constants.rb

@@ -1,7 +1,7 @@
 # match expressions when taps are given as ARGS, e.g. someuser/sometap
 HOMEBREW_TAP_ARGS_REGEX = %r{^([\w-]+)/(homebrew-)?([\w-]+)$}
 # match taps' formulae, e.g. someuser/sometap/someformula
-HOMEBREW_TAP_FORMULA_REGEX = %r{^([\w-]+)/([\w-]+)/([\w+-.]+)$}
+HOMEBREW_TAP_FORMULA_REGEX = %r{^([\w-]+)/([\w-]+)/([\w+-.@]+)$}
 # match core's formulae, e.g. homebrew/homebrew/someformula
 HOMEBREW_CORE_FORMULA_REGEX = %r{^homebrew/homebrew/([\w+-.]+)$}i
 # match taps' directory paths, e.g. HOMEBREW_LIBRARY/Taps/someuser/sometap