[bundler/bundler] Do `require "rubygems"` only when needed

This require causes circular require.

```
$ touch empty_file

$ RUBYGEMS_GEMDEPS=empty_file ./local/bin/ruby -w -e ''
/home/mame/work/ruby/local/lib/ruby/2.7.0/rubygems/core_ext/kernel_require.rb:92: warning: /home/mame/work/ruby/local/lib/ruby/2.7.0/rubygems/core_ext/kernel_require.rb:92: warning: loading in progress, circular require considered harmful - /home/mame/work/ruby/local/lib/ruby/2.7.0/rubygems.rb
	from <internal:gem_prelude>:1:in  `<internal:gem_prelude>'
	from <internal:gem_prelude>:1:in  `require'
	from /home/mame/work/ruby/local/lib/ruby/2.7.0/rubygems.rb:1417:in  `<top (required)>'
	from /home/mame/work/ruby/local/lib/ruby/2.7.0/rubygems.rb:1203:in  `use_gemdeps'
	from /home/mame/work/ruby/local/lib/ruby/2.7.0/rubygems/user_interaction.rb:47:in  `use_ui'
	from /home/mame/work/ruby/local/lib/ruby/2.7.0/rubygems.rb:1204:in  `block in use_gemdeps'
	from /home/mame/work/ruby/local/lib/ruby/2.7.0/rubygems/core_ext/kernel_require.rb:92:in  `require'
	from /home/mame/work/ruby/local/lib/ruby/2.7.0/rubygems/core_ext/kernel_require.rb:92:in  `require'
	from /home/mame/work/ruby/local/lib/ruby/2.7.0/bundler.rb:11:in  `<top (required)>'
	from /home/mame/work/ruby/local/lib/ruby/2.7.0/bundler.rb:11:in  `require_relative'
	from /home/mame/work/ruby/local/lib/ruby/2.7.0/bundler/rubygems_integration.rb:3:in  `<top (required)>'
	from /home/mame/work/ruby/local/lib/ruby/2.7.0/rubygems/core_ext/kernel_require.rb:92:in  `require'
	from /home/mame/work/ruby/local/lib/ruby/2.7.0/rubygems/core_ext/kernel_require.rb:92:in  `require'
```

https://github.com/bundler/bundler/commit/c7c5bcea92

lib/bundler/rubygems_integration.rb

@@ -1,6 +1,6 @@
 # frozen_string_literal: true
 
-require "rubygems"
+require "rubygems" unless defined?(Gem)
 
 module Bundler
   class RubygemsIntegration