Fix calculation of cop department for nested departments

This fixes an issue I ran into with rubocop-i18n. This gem provides two
nested departments, I18n/RailsI18n and I18n/GetText. The intent is to
enable one and disable the other. However, disabling a department would
not disable its cops.

I tracked this issue down to how RuboCop::Config calculates a cops
department before looking at that department's configuration: It only
considers the part before the first `/`.

This commit changes this method by considering instead all parts before
the last `/`.

changelog/fix_fix_calculation_of_cop_department_for.md

@@ -0,0 +1 @@
+* [#9258](https://github.com/rubocop-hq/rubocop/pull/9258): Fix calculation of cop department for nested departments. ([@mvz][])

lib/rubocop/config.rb

@@ -292,10 +292,10 @@ def enable_cop?(qualified_cop_name, cop_options)
     end
 
     def department_of(qualified_cop_name)
-      cop_department, cop_name = qualified_cop_name.split('/')
-      return nil if cop_name.nil?
+      *cop_department, _ = qualified_cop_name.split('/')
+      return nil if cop_department.empty?
 
-      self[cop_department]
+      self[cop_department.join('/')]
     end
   end
 end

spec/rubocop/config_spec.rb

@@ -781,6 +781,22 @@ def cop_enabled(cop_class)
       end
     end
 
+    context 'when an nested cop department is disabled' do
+      context 'but an individual cop is enabled' do
+        let(:hash) do
+          {
+            'Foo/Bar' => { 'Enabled' => false },
+            'Foo/Bar/BazCop' => { 'Enabled' => true }
+          }
+        end
+
+        it 'still disables the cop' do
+          cop_class = 'Foo/Bar/BazCop'
+          expect(cop_enabled(cop_class)).to be false
+        end
+      end
+    end
+
     context 'when an entire cop department is enabled' do
       context 'but an individual cop is disabled' do
         let(:hash) do