Move "path or URL" tap string to global constant.

Library/Homebrew/formula.rb

@@ -540,7 +540,7 @@ class Formula
     elsif core_formula?
       "Homebrew/homebrew"
     else
-      "path or URL"
+      HOMEBREW_PATH_URL_TAP
     end
   end
 

Library/Homebrew/tap_constants.rb

@@ -8,3 +8,5 @@ HOMEBREW_TAP_DIR_REGEX = %r{#{Regexp.escape(HOMEBREW_LIBRARY.to_s)}/Taps/([\w-]+
 HOMEBREW_TAP_PATH_REGEX = Regexp.new(HOMEBREW_TAP_DIR_REGEX.source + %r{/(.*)}.source)
 # match the default brew-cask tap e.g. Caskroom/cask
 HOMEBREW_CASK_TAP_FORMULA_REGEX = %r{^(Caskroom)/(cask)/([\w+-.]+)$}
+# the tap name used for formulae installed from paths or URLs
+HOMEBREW_PATH_URL_TAP = "path or URL"