fixup! add tests relevant to handling extensions to external types

Author: theMomax

Tests/SwiftDocCTests/Infrastructure/ReferenceResolverTests.swift

@@ -632,9 +632,9 @@ private extension Collection where Element == RenderBlockContent {
         var headingLevel: Int = 1
 
         guard let headingIndex = self.firstIndex(where: { element in
-            if case let .heading(elementLevel, elementHeading, _) = element {
-                headingLevel = elementLevel
-                return heading == elementHeading
+            if case let .heading(value) = element {
+                headingLevel = value.level
+                return heading == value.text
             }
             return false
         }) else {
@@ -644,17 +644,17 @@ private extension Collection where Element == RenderBlockContent {
         let contentStart = self.index(after: headingIndex)
 
         return Slice(base: self, bounds: contentStart..<(self[contentStart...].firstIndex(where: { element in
-            if case let .heading(elementLevel, _, _) = element {
-                return elementLevel <= headingLevel
+            if case let .heading(value) = element {
+                return value.level <= headingLevel
             }
             return false
         }) ?? self.endIndex))
     }
 
     func listItems() -> [RenderBlockContent.ListItem] {
         self.compactMap { block -> [RenderBlockContent.ListItem]? in
-            if case let .unorderedList(items) = block {
-                return items
+            if case let .unorderedList(value) = block {
+                return value.items
             }
             return nil
         }.flatMap({ $0 })
@@ -664,11 +664,11 @@ private extension Collection where Element == RenderBlockContent {
 private extension RenderBlockContent.ListItem {
     func firstReference() -> RenderReferenceIdentifier? {
         self.content.compactMap { block in
-            guard case let .paragraph(inlineContent) = block else {
+            guard case let .paragraph(value) = block else {
                 return nil
             }
 
-            return inlineContent.compactMap { content in
+            return value.inlineContent.compactMap { content in
                 guard case let .reference(identifier, _, _, _) = content else {
                     return nil
                 }