Only consider migration classes for MigrationClassName

[sunny]

Adds a fix to #644 when calling using multiple classes inside a migration.

Currently, when using the pattern of redefining ActiveRecord classes inside the migration in order not to depend on the app’s classes:

class UpdateLocaleOnArticles < ActiveRecord::Migration[6.1]
  class Article < ActiveRecord::Base
  end

  def up
    Article.update_all(locale: "fr")
  end
end

Produces:

db/migrate/20211025103032_update_locale_on_articles.rb:4:9: C: [Correctable] Rails/MigrationClassName: Replace with UpdateLocaleOnArticles that matches the file name.
  class Article < ActiveRecord::Base
        ^^^^^^^

Which autocorrects to:

db/migrate/20211025103032_update_locale_on_articles.rb:4:9: C: [Correctable] Rails/MigrationClassName: Replace with UpdateLocaleOnArticles that matches the file name.
  class UpdateLocaleOnArticles < ActiveRecord::Base
        ^^^^^^^^^^^^^^^^^^^^^^

This fix ignores classes included inside the migration.

---

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[koic]

Nice!