[ruby/rubygems] Fix native extension loading in newgem template for RHEL-based systems

Add fallback to `require` when `require_relative` fails to load native
extensions. This addresses an issue on RHEL-based Linux distributions
where Ruby scripts and built native extension shared libraries are
installed in separate directories.

ruby/rubygems@68599bd107

Author: yoshoku

lib/bundler/templates/newgem/lib/newgem.rb.tt

@@ -2,7 +2,7 @@
 
 require_relative "<%= File.basename(config[:namespaced_path]) %>/version"
 <%- if config[:ext] -%>
-require_relative "<%= File.basename(config[:namespaced_path]) %>/<%= config[:underscored_name] %>"
+require "<%= File.basename(config[:namespaced_path]) %>/<%= config[:underscored_name] %>"
 <%- end -%>
 
 <%- config[:constant_array].each_with_index do |c, i| -%>

spec/bundler/commands/newgem_spec.rb

@@ -1681,6 +1681,13 @@ def create_temporary_dir(dir)
         expect(bundled_app("#{gem_name}/ext/#{gem_name}/#{gem_name}.c")).to exist
       end
 
+      it "generates native extension loading code" do
+        expect(bundled_app("#{gem_name}/lib/#{gem_name}.rb").read).to include(<<~RUBY)
+          require_relative "test_gem/version"
+          require "#{gem_name}/#{gem_name}"
+        RUBY
+      end
+
       it "includes rake-compiler, but no Rust related changes" do
         expect(bundled_app("#{gem_name}/Gemfile").read).to include('gem "rake-compiler"')