Ensure only directories exist in Rails default load paths

[aeroastro]

Motivation / Background

As commented at #37011 (comment),
files such as app/README.md should not be in the autoload paths.

Detail

This PR removes files from the default load paths set up by the Rails framework.

Additional information

Although File.stub(:exist?, true) in existing test has been changed to File.stub(:directory?, true),
it is obvious that this does not break the objectives of the test.

Checklist

Before submitting the PR make sure the following are checked:

• This Pull Request is related to one change. Changes that are unrelated should be opened in separate PRs.
• Commit message has a detailed description of what changed and why. If this PR fixes a related issue include it in the commit message. Ex: [Fix #issue-number]
• Tests are added or updated if you fix a bug or add a feature.
• CHANGELOG files are updated for the changed libraries if there is a behavior change or additional feature. Minor bug fixes and documentation changes should not be included.

[aeroastro]

All the tests have successfully passed 🐱

Currently, since these paths (autoload_paths, autoload_once_paths, eager_load_paths, and load_paths) are used with File.directory? or Dir.exist?, the changes introduced in this PR does not break the behavior of public interfaces of Rails framework.

rails/railties/lib/rails/application/bootstrap.rb

Lines 105 to 108 in 5034b08

	ActiveSupport::Dependencies.autoload_once_paths.freeze
	ActiveSupport::Dependencies.autoload_once_paths.uniq.each do |path|
	# Zeitwerk only accepts existing directories in `push_dir`.
	next unless File.directory?(path)

rails/railties/lib/rails/application/finisher.rb

Lines 25 to 28 in 5034b08

	ActiveSupport::Dependencies.autoload_paths.freeze
	ActiveSupport::Dependencies.autoload_paths.uniq.each do |path|
	# Zeitwerk only accepts existing directories in `push_dir`.
	next unless File.directory?(path)

rails/railties/lib/rails/zeitwerk_checker.rb

Lines 5 to 12 in 5034b08

	def self.check
	Zeitwerk::Loader.eager_load_all
	autoloaded = ActiveSupport::Dependencies.autoload_paths + ActiveSupport::Dependencies.autoload_once_paths
	eager_loaded = ActiveSupport::Dependencies._eager_load_paths.to_a
	unchecked = autoloaded - eager_loaded
	unchecked.select! { |dir| Dir.exist?(dir) && !Dir.empty?(dir) }

rails/railties/lib/rails/engine.rb

Lines 572 to 577 in 5034b08

	initializer :set_load_path, before: :bootstrap_hook do |app|
	_all_load_paths(app.config.add_autoload_paths_to_load_path).reverse_each do |path|
	$LOAD_PATH.unshift(path) if File.directory?(path)
	end
	$LOAD_PATH.uniq!
	end

[fxn]

Looks good. I'll edit a bit the CHANGELOG later (first sentence lacks a final period, in a Rails CHANGELOG you do not need to say you are talking about the Rails framework, ...).

Thanks!