Utilities/Sphinx: Restore html search index link anchors

Since !8305 (merged), anchors in links to cmake domain objects generated in html search results were missing their object type prefix, and thus did not link to the object.

Restore our get_objects implementation's second tuple entry to what we used prior to that commit. This matches what Sphinx's builtin python, rst, and javascript modes do.

Fixes: #25067 (closed)
Backport: release