FIX: migrations-tooling CLI didn't work anymore (#29777)

The previous approach of splitting Thor commands into multiple files caused problems when the same method name was used in multiple commands.

This also loads the Rails environment only for commands that need it. That makes the CLI boot faster for most commands or when the help should be shown. That's also why we can't use `Rails.root` in the CLI.

migrations/lib/cli/import_command.rb

@@ -1,15 +1,18 @@
 # frozen_string_literal: true
 
-module Migrations::CLI::ImportCommand
-  def self.included(thor)
-    thor.class_eval do
-      desc "import", "Import a file"
-      def import
-        require "extralite"
+require "extralite"
 
-        puts "Importing into Discourse #{Discourse::VERSION::STRING}"
-        puts "Extralite SQLite version: #{Extralite.sqlite3_version}"
-      end
+module Migrations::CLI
+  class ImportCommand
+    def initialize(options)
+      @options = options
+    end
+
+    def execute
+      ::Migrations.load_rails_environment
+
+      puts "Importing into Discourse #{Discourse::VERSION::STRING}"
+      puts "Extralite SQLite version: #{Extralite.sqlite3_version}"
     end
   end
 end