Allow tasks to be loaded from nested task directories

Before this commit, only tasks under `app/tasks` will be loaded in
development. After this commit, one level of nesting is supported.

app/models/maintenance_tasks/task.rb

@@ -175,8 +175,23 @@ def load_constants
         namespace = MaintenanceTasks.tasks_module.safe_constantize
         return unless namespace
 
-        namespace.constants.map { |constant| namespace.const_get(constant) }
+        constants = []
+
+        namespace.constants.each do |constant_name|
+          constant = namespace.const_get(constant_name)
+
+          case constant
+          when Module
+            constant.constants.each do |nested_constant_name|
+              constants << constant.const_get(nested_constant_name)
+            end
+          when Class
+            constants << constant
+          end
+        end
       end
+
+      constants
     end
 
     # The contents of a CSV file to be processed by a Task.

test/dummy/app/tasks/maintenance/nested/nested_task.rb

@@ -0,0 +1,11 @@
+# frozen_string_literal: true
+
+module Maintenance
+  module Nested
+    class NestedTask < MaintenanceTasks::Task
+      def process(rows)
+        # Task only exists to verify correct loading of tasks within subfolders
+      end
+    end
+  end
+end

test/models/maintenance_tasks/task_data_index_test.rb

@@ -12,6 +12,7 @@ class TaskDataIndexTest < ActiveSupport::TestCase
         "Maintenance::EnqueueErrorTask",
         "Maintenance::ErrorTask",
         "Maintenance::ImportPostsTask",
+        "Maintenance::Nested::NestedTask",
         "Maintenance::NoCollectionTask",
         # duplicate due to fixtures containing two active runs of this task
         "Maintenance::NoCollectionTask",

test/models/maintenance_tasks/task_test.rb

@@ -12,6 +12,7 @@ class TaskTest < ActiveSupport::TestCase
         "Maintenance::EnqueueErrorTask",
         "Maintenance::ErrorTask",
         "Maintenance::ImportPostsTask",
+        "Maintenance::Nested::NestedTask",
         "Maintenance::NoCollectionTask",
         "Maintenance::ParamsTask",
         "Maintenance::TestTask",

test/system/maintenance_tasks/tasks_test.rb

@@ -27,6 +27,7 @@ class TasksTest < ApplicationSystemTestCase
         "Maintenance::CancelledEnqueueTask\nNew",
         "Maintenance::EnqueueErrorTask\nNew",
         "Maintenance::ErrorTask\nNew",
+        "Maintenance::Nested::NestedTask\nNew",
         "Maintenance::ParamsTask\nNew",
         "Maintenance::TestTask\nNew",
         "Maintenance::UpdatePostsInBatchesTask\nNew",