Add missing fileutils require

[deivid-rodriguez]

Description:

On my system, the error was being hidden by the presence of a YARD rubygems plugin that was providing the require and making things work.

However, on a pristine system, gem outdated would fail with

 ERROR:  While executing gem ... (NameError)
    uninitialized constant Gem::Source::FileUtils
Did you mean?  FileTest

Fixes #3894.

Tasks:

• Describe the problem / feature
• Write tests
• Write code to solve the problem
• Get code review from coworkers / friends

I will abide by the code of conduct.

[FrankFang]

Hey guys, I notice another "FileUtils.mkdir_p" at line 151 of source.rb

Why not add require "fileutils" before it ?

Installing ruby-debug-ide 0.7.2 with native extensions
Gem::Ext::BuildError: ERROR: Failed to build gem native extension.

    current directory: /home/aaa/bbb/vendor/bundle/ruby/3.0.0/gems/ruby-debug-ide-0.7.2/ext
/usr/local/bin/ruby mkrf_conf.rb
Installing base gem
uninitialized constant Gem::Source::FileUtils
Did you mean?  FileTest
/usr/local/lib/ruby/3.0.0/rubygems/source.rb:151:in `fetch_spec'
  /usr/local/lib/ruby/3.0.0/rubygems/resolver/api_specification.rb:91:in `spec'
  /usr/local/lib/ruby/3.0.0/rubygems/resolver/installer_set.rb:77:in `add_always_install'
  /usr/local/lib/ruby/3.0.0/rubygems/dependency_installer.rb:321:in `resolve_dependencies'
  /usr/local/lib/ruby/3.0.0/rubygems/dependency_installer.rb:228:in `install'
  mkrf_conf.rb:31:in `rescue in <main>'
  mkrf_conf.rb:24:in `<main>'

rake failed, exit code 1

rails version: Rails 6.1.0
ruby version: ruby 3.0.0preview2

[FrankFang]

I see, it was fixed at the master branch https://github.com/rubygems/rubygems/blob/master/lib/rubygems/source.rb#L157