which: allow <formulae> args

Also clean-up this external command in preparation
for becoming a built-in command.

Make which_versions available in Homebrew module so
that it can be used by other commands in the future.

Library/Contributions/examples/brew-which.rb

 require 'extend/pathname'
 
-REAL_CELLAR = HOMEBREW_CELLAR.realpath
 
-class String
-  def starts_with? prefix
-    prefix = prefix.to_s
-    self[0, prefix.length] == prefix
-  end
-end
+module Homebrew extend self
+  def which_versions which_brews=nil
+    brew_links = Array.new
+    version_map = Hash.new
+
+    real_cellar = HOMEBREW_CELLAR.realpath
+
+    # paths=%w[bin sbin etc lib include share].collect {|d| HOMEBREW_PREFIX+d}
+    paths=%w[bin].collect {|d| HOMEBREW_PREFIX+d}
+
+    paths.each do |path|
+      path.find do |path|
+        next unless path.symlink? && path.resolved_path_exists?
+        brew_links << Pathname.new(path.realpath)
+      end
+    end
 
+    brew_links = brew_links.collect{|p|p.relative_path_from(real_cellar).to_s}.reject{|p|p.start_with?("../")}
 
-def audit
-  brew_links = Array.new
-  version_map = Hash.new
+    brew_links.each do |p|
+      parts = p.split("/")
+      next if parts.count < 2 # Shouldn't happen for normally installed brews
+      brew = parts.shift
+      version = parts.shift
 
-  # paths=%w[bin sbin etc lib include share].collect {|d| HOMEBREW_PREFIX+d}
-  paths=%w[bin].collect {|d| HOMEBREW_PREFIX+d}
+      next unless which_brews.include? brew if which_brews
 
-  paths.each do |path|
-    path.find do |path|
-      next unless path.symlink? && path.resolved_path_exists?
-      brew_links << Pathname.new(path.realpath)
+      versions = version_map[brew] || []
+      versions << version unless versions.include? version
+      version_map[brew] = versions
     end
+
+    return version_map
   end
 
-  brew_links = brew_links.collect{|p|p.relative_path_from(REAL_CELLAR).to_s}.reject{|p|p.starts_with?("../")}
-  brew_links.each do |p|
-    parts = p.split("/")
-    next if parts.count < 2 # Shouldn't happen
-    brew = parts.shift
-    version = parts.shift
+  def which
+    which_brews = ARGV.named.empty? ? nil : ARGV.named
 
-    versions = version_map[brew] || []
-    versions << version unless versions.include? version
-    version_map[brew] = versions
+    brews = which_versions which_brews
+    brews.keys.sort.each do |b|
+      puts "#{b}: #{brews[b].sort*' '}"
+    end
+    puts
   end
-
-  return version_map
 end
 
-brews = audit
-brews.keys.sort.each do |b|
-  puts "#{b}: #{brews[b].sort*' '}"
-end
-puts
+Homebrew.which