Change a way ActiveRecord's config is prepared for rake tasks

In commit d1d7c86d I moved setting migrations paths into activerecord's
railtie to remove Rails dependency on databases.rake. However, it
introduced a regression, ENGINE_PATH was not available at the moment, so
engine's migrations where not added properly to paths. Fix was added
at 97a4a771, but it changes a way things work from using ENGINE_PATH to
APP_RAKEFILE. Additionally, the config runs when the code loads, while
previously it ran in the db:load_config rake task

In order to make it more in pair with original version this commit
changes the config to run only on load_config task. This code uses the
fact that defining a task in rake does not overwrite, but only appends.

It also allows to get back to checking for ENGINE_PATH

activerecord/lib/active_record/railtie.rb

@@ -37,16 +37,20 @@ class Railtie < Rails::Railtie # :nodoc:
     rake_tasks do
       require "active_record/base"
 
-      ActiveRecord::Tasks::DatabaseTasks.env = Rails.env
-      ActiveRecord::Tasks::DatabaseTasks.db_dir = Rails.application.config.paths["db"].first
-      ActiveRecord::Tasks::DatabaseTasks.seed_loader = Rails.application
-      ActiveRecord::Tasks::DatabaseTasks.database_configuration = Rails.application.config.database_configuration
-      ActiveRecord::Tasks::DatabaseTasks.migrations_paths = Rails.application.paths['db/migrate'].to_a
-      ActiveRecord::Tasks::DatabaseTasks.fixtures_path = File.join Rails.root, 'test', 'fixtures'
-
-      if defined?(APP_RAKEFILE) && engine = Rails::Engine.find(find_engine_path(APP_RAKEFILE))
-        if engine.paths['db/migrate'].existent
-          ActiveRecord::Tasks::DatabaseTasks.migrations_paths += engine.paths['db/migrate'].to_a
+      namespace :db do
+        task :load_config do
+          ActiveRecord::Tasks::DatabaseTasks.env = Rails.env
+          ActiveRecord::Tasks::DatabaseTasks.db_dir = Rails.application.config.paths["db"].first
+          ActiveRecord::Tasks::DatabaseTasks.seed_loader = Rails.application
+          ActiveRecord::Tasks::DatabaseTasks.database_configuration = Rails.application.config.database_configuration
+          ActiveRecord::Tasks::DatabaseTasks.migrations_paths = Rails.application.paths['db/migrate'].to_a
+          ActiveRecord::Tasks::DatabaseTasks.fixtures_path = File.join Rails.root, 'test', 'fixtures'
+
+          if defined?(ENGINE_PATH) && engine = Rails::Engine.find(ENGINE_PATH)
+            if engine.paths['db/migrate'].existent
+              ActiveRecord::Tasks::DatabaseTasks.migrations_paths += engine.paths['db/migrate'].to_a
+            end
+          end
         end
       end