Leverage ruby feature to warn when requiring default gems not included in the bundle that will be turned into bundled gems in the future

[hsbt]

What was the end-user or developer problem that led to this PR?

When users load rexml after upgrading Ruby version, they got LoadError of rexml. Because rexml is now bundled gems, not default gems that is shipped with under RbConfig::CONFIG["rubylibdir"].

I want to eliminate this surprised situation.

What is your fix for the problem, implemented in this PR?

I added warning feature for bundled gems and default gems. When users did load default gems that are listed on our plan of bundled gems, we can notice to them.

Notification message is like this.

>> require "csv"
csv is not part of the default gems since Ruby 3.4.0. Add it to your Gemfile.
=> true

and I also notice bundled gems like this:

>> require "net-imap"
net-imap is not part of the default gems since Ruby 3.1.0. Add it to your Gemfile.
/Users/hsbt/.local/share/rbenv/versions/master/lib/ruby/3.3.0+0/bundler/rubygems_integration.rb:261:in `require': cannot load such file -- net-imap (LoadError)

I believe this feature is helpful for Bundler users.

also see https://bugs.ruby-lang.org/issues/19776

TODO

I didn't implement to notice about gemspec. If foo gem depends on csv intenally, This PR warns only about csv and Gemfile. We should also warn about foo.gemspec.

Make sure the following tasks are checked

• Describe the problem / feature
• Write tests for features and bug fixes
• Write code to solve the problem
• Make sure you follow the current code style and write meaningful commit messages without tags

[hsbt]

Gem::BUNDLED_GEMS is here: https://github.com/ruby/ruby/blob/98bd5a71f2a01db450e2a2722d1938fdb9e784e9/lib/bundled_gems.rb

[headius]

@hsbt I think we could just include the same bundled_gems.rb in JRuby stdlib and get this functionality yes?

Of the 3.4 gems, only two have extensions: nkf, which we've fixed in ruby/nkf#15, and syslog, which JRuby provides with FFI (we should get that into the gem). I think they all would be good for warnings.