Remove global methods from formulae

Removes any global methods from formulae, and moves #kext_prefix (which
seems to be at least somewhat abstractable) into the Formula class. The
only formula with global methods is now aspell; it (and its generating
script in contrib) has been changed to prefix that method with
`aspell_`, to minimize the risk of name collisions.

Closes Homebrew/homebrew#19331.
Closes Homebrew/homebrew#19343.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>

Library/Contributions/cmd/brew-aspell-dictionaries

@@ -40,7 +40,7 @@ EOF
 done
 
 cat <<EOF >> $brew_formulae_tmp_file
-def available_languages
+def aspell_available_languages
   %w(${langs})
 end
 # END generated with `basename $0`

Library/Homebrew/formula.rb

@@ -165,6 +165,8 @@ class Formula
   def sbin;    prefix+'sbin'    end
   def share;   prefix+'share'   end
 
+  def kext_prefix; prefix+'Library/Extensions' end
+
   # configuration needs to be preserved past upgrades
   def etc; HOMEBREW_PREFIX+'etc' end
   # generally we don't want var stuff inside the keg