fix(internal/godocfx): add TOC element for module root package (#3599)

Author: tbpg

internal/godocfx/parse.go

@@ -339,9 +339,16 @@ func buildTOC(mod string, pis []pkgInfo, extraFiles []extraFile) tableOfContents
 	toc := tableOfContents{}
 
 	modTOC := &tocItem{
-		UID:  mod, // Assume the module root has a package.
+		UID:  mod,
 		Name: mod,
 	}
+
+	// Assume the module root has a package.
+	modTOC.addItem(&tocItem{
+		UID:  mod,
+		Name: mod,
+	})
+
 	for _, ef := range extraFiles {
 		modTOC.addItem(&tocItem{
 			Href: ef.dstRelativePath,

internal/godocfx/testdata/golden/toc.yml

@@ -2,5 +2,7 @@
 - uid: cloud.google.com/go/storage
   name: cloud.google.com/go/storage
   items:
+  - uid: cloud.google.com/go/storage
+    name: cloud.google.com/go/storage
   - name: README
     href: pkg-readme.md