Refactor command handling code

Don’t rely on having external commands always present in the PATH in
order to find them. Instead, provide an accessory method to Tap so
they can be added and used when needed.

While we’re here, do some general refactoring and cleanup of the
command code in these places.

Library/Homebrew/cmd/command.rb

@@ -8,15 +8,16 @@ module Homebrew
 
   def command
     abort "This command requires a command argument" if ARGV.empty?
-    cmd = ARGV.first
-    cmd = HOMEBREW_INTERNAL_COMMAND_ALIASES.fetch(cmd, cmd)
 
-    if (path = Commands.path(cmd))
-      puts path
-    elsif (path = which("brew-#{cmd}") || which("brew-#{cmd}.rb"))
-      puts path
-    else
-      odie "Unknown command: #{cmd}"
-    end
+    cmd = HOMEBREW_INTERNAL_COMMAND_ALIASES.fetch(ARGV.first, ARGV.first)
+
+    path = Commands.path(cmd)
+
+    cmd_paths = PATH.new(ENV["PATH"]).append(Tap.cmd_directories) unless path
+    path ||= which("brew-#{cmd}", cmd_paths)
+    path ||= which("brew-#{cmd}.rb", cmd_paths)
+
+    odie "Unknown command: #{cmd}" unless path
+    puts path
   end
 end

Library/Homebrew/cmd/commands.rb

@@ -9,27 +9,30 @@ module Homebrew
 
   def commands
     if ARGV.include? "--quiet"
-      cmds = internal_commands + external_commands
+      cmds = internal_commands
+      cmds += external_commands
       cmds += internal_developer_commands
       cmds += HOMEBREW_INTERNAL_COMMAND_ALIASES.keys if ARGV.include? "--include-aliases"
       puts Formatter.columns(cmds.sort)
-    else
-      # Find commands in Homebrew/cmd
-      puts "Built-in commands"
-      puts Formatter.columns(internal_commands.sort)
-
-      # Find commands in Homebrew/dev-cmd
-      puts
-      puts "Built-in developer commands"
-      puts Formatter.columns(internal_developer_commands.sort)
-
-      # Find commands in the path
-      unless (exts = external_commands).empty?
-        puts
-        puts "External commands"
-        puts Formatter.columns(exts)
-      end
+      return
     end
+
+    # Find commands in Homebrew/cmd
+    puts "Built-in commands"
+    puts Formatter.columns(internal_commands.sort)
+
+    # Find commands in Homebrew/dev-cmd
+    puts
+    puts "Built-in developer commands"
+    puts Formatter.columns(internal_developer_commands.sort)
+
+    exts = external_commands
+    return if exts.empty?
+
+    # Find commands in the PATH
+    puts
+    puts "External commands"
+    puts Formatter.columns(exts)
   end
 
   def internal_commands
@@ -41,11 +44,13 @@ module Homebrew
   end
 
   def external_commands
-    paths.each_with_object([]) do |path, cmds|
+    cmd_paths = PATH.new(ENV["PATH"]).append(Tap.cmd_directories)
+    cmd_paths.each_with_object([]) do |path, cmds|
       Dir["#{path}/brew-*"].each do |file|
         next unless File.executable?(file)
         cmd = File.basename(file, ".rb")[5..-1]
-        cmds << cmd unless cmd.include?(".")
+        next if cmd.include?(".")
+        cmds << cmd
       end
     end.sort
   end