Move ID of the content section to AccordionSection

This is a stable piece of information, so it makes sense that it's part of the `AccordionSection`'s property.

It currently requires `getIdentifier` to remain within `Accordion`, but hopefully not for long, as the aim would be for the methods that use `getIdentifier` to use APIs from `AccordionSection` rather than access the DOM.

packages/govuk-frontend/src/govuk/components/accordion/accordion-section.mjs

@@ -14,9 +14,15 @@ export class AccordionSection extends Component {
 
   /**
    * @param {Element} $root - The root of the section
+   * @param {{accordionId: string, index: number}} config - Configuration for the section
    */
-  constructor($root) {
+  constructor($root, config) {
     super($root)
+    // We do not need all the cruft of ConfigurableComponent as there's no defaults
+    // or need to read config from the root's HTML attributes
+    // If anything we actually don't need to store the config at all during the section's lifetime
+    // only the contentId
+    this.contentId = `${config.accordionId}-content-${config.index + 1}`
 
     this.$header = this.findRequiredElement(
       `.govuk-accordion__section-header`,
@@ -34,6 +40,7 @@ export class AccordionSection extends Component {
     // However, I'm not sure we originally intended to check whether the content element
     // is present every time we toggle a section.
     // If that's the case, we can always wrap this in a setter
+    // TODO: Update to use the ID computed for the `aria-controls` of the `<button>`
     this.$content = this.findRequiredElement(
       `.govuk-accordion__section-content`,
       `Section content (\`<div class="govuk-accordion__section-content">\`)`

packages/govuk-frontend/src/govuk/components/accordion/accordion.mjs

@@ -141,13 +141,16 @@ export class Accordion extends ConfigurableComponent {
    * @private
    */
   initSectionHeaders() {
-    this.$sections.forEach(($section, i) => {
-      const section = new AccordionSection($section)
+    this.$sections.forEach(($section, index) => {
+      const section = new AccordionSection($section, {
+        accordionId: this.$root.id,
+        index
+      })
       // Cache the AccordionSection for future retrieval
       this.sections.set($section, section)
 
       // Set header attributes
-      this.constructHeaderMarkup(section, i)
+      this.constructHeaderMarkup(section, index)
       this.setExpanded(this.isExpanded($section), $section)
 
       // Handle events
@@ -375,9 +378,10 @@ export class Accordion extends ConfigurableComponent {
    * @returns {string | undefined | null} Identifier for section
    */
   getIdentifier($section) {
-    const $button = $section.querySelector(`.${sectionButtonClass}`)
-
-    return $button?.getAttribute('aria-controls')
+    // TODO: Temporary, this should be lifted once the accordion
+    // all section related features as within `AccordionSection`
+    const section = this.sections.get($section)
+    return section?.contentId
   }
 
   /**