outdated: check if formula needs to be migrated.

Closes Homebrew/homebrew#42938.

Library/Homebrew/cmd/outdated.rb

 require "formula"
 require "keg"
+require "migrator"
 
 module Homebrew
   def outdated
@@ -16,6 +17,13 @@ module Homebrew
     formulae.map do |f|
       all_versions = []
       older_or_same_tap_versions = []
+
+      if f.oldname && !f.rack.exist?
+        if Pathname.new("#{HOMEBREW_CELLAR}/#{f.oldname}").exist?
+          raise Migrator::MigrationNeededError.new(f)
+        end
+      end
+
       f.rack.subdirs.each do |dir|
         keg = Keg.new dir
         version = keg.version

Library/Homebrew/migrator.rb

@@ -4,6 +4,15 @@ require "tab"
 require "tap_migrations"
 
 class Migrator
+  class MigrationNeededError < RuntimeError
+    def initialize(formula)
+      super <<-EOS.undent
+        #{formula.oldname} was renamed to #{formula.name} and needs to be migrated.
+        Please run `brew migrate #{formula.oldname}`
+      EOS
+    end
+  end
+
   class MigratorNoOldnameError < RuntimeError
     def initialize(formula)
       super "#{formula.name} doesn't replace any formula."