Add @ to valid package name characters (#18862)

NodeJS formulae are now named node@<major_version>

E.g. https://github.com/Homebrew/homebrew-core/blob/master/Formula/node@6.rb

lib/ansible/modules/packaging/os/homebrew.py

@@ -179,6 +179,7 @@ class Homebrew(object):
         \+                  # plusses
         -                   # dashes
         :                   # colons (for URLs)
+        @                   # at-sign        
     '''
 
     INVALID_PATH_REGEX        = _create_regex_group(VALID_PATH_CHARS)