[rubygems/rubygems] Make sure to require "rubygems" explicitly

This is also done in bundler/lib/bundler/rubygems_integration.rb, but
bundler/lib/bundler.rb loads this file before it.

https://github.com/rubygems/rubygems/commit/8840d8507b

lib/bundler/rubygems_ext.rb

@@ -2,6 +2,8 @@
 
 require "pathname"
 
+require "rubygems" unless defined?(Gem)
+
 require "rubygems/specification"
 
 # We can't let `Gem::Source` be autoloaded in the `Gem::Specification#source`

spec/bundler/runtime/requiring_spec.rb

@@ -0,0 +1,15 @@
+# frozen_string_literal: true
+
+RSpec.describe "Requiring bundler" do
+  it "takes care of requiring rubygems when entrypoint is bundler/setup" do
+    sys_exec("#{Gem.ruby} -I#{lib_dir} -rbundler/setup -e'puts true'", :env => { "RUBYOPT" => opt_add("--disable=gems", ENV["RUBYOPT"]) })
+
+    expect(last_command.stdboth).to eq("true")
+  end
+
+  it "takes care of requiring rubygems when requiring just bundler" do
+    sys_exec("#{Gem.ruby} -I#{lib_dir} -rbundler -e'puts true'", :env => { "RUBYOPT" => opt_add("--disable=gems", ENV["RUBYOPT"]) })
+
+    expect(last_command.stdboth).to eq("true")
+  end
+end

spec/bundler/runtime/setup_spec.rb

@@ -1506,12 +1506,6 @@ end
       expect(err).to include "private method `require'"
     end
 
-    it "takes care of requiring rubygems" do
-      sys_exec("#{Gem.ruby} -I#{lib_dir} -rbundler/setup -e'puts true'", :env => { "RUBYOPT" => opt_add("--disable=gems", ENV["RUBYOPT"]) })
-
-      expect(last_command.stdboth).to eq("true")
-    end
-
     it "memoizes initial set of specs when requiring bundler/setup, so that even if further code mutates dependencies, Bundler.definition.specs is not affected" do
       install_gemfile <<~G
         source "#{file_uri_for(gem_repo1)}"