[rubygems/rubygems] Avoid more warnings when using RubyGems with old Bundler

We were only avoiding them when the RUBYGEMS_GEMDEPS variable is used.
Avoid the warnings in general, whenever the entrypoint to Bundler is
`require`.

8683faef36

lib/rubygems/core_ext/kernel_require.rb

@@ -64,8 +64,11 @@ module Kernel
           rp
         end
 
-        Kernel.send(:gem, name, Gem::Requirement.default_prerelease) unless
-          resolved_path
+        next if resolved_path
+
+        Kernel.send(:gem, name, Gem::Requirement.default_prerelease)
+
+        Gem.load_bundler_extensions(Gem.loaded_specs[name].version) if name == "bundler"
 
         next
       end