Fix issues with the singlehtml builder's objects.inv file

[jdillard]

Purpose

index.html doesn't appear included in the objects.inv file for the singlehtml builder, making it break intersphinx links where the webserver doesn't redirect for a missing index.html in the URL.

The objects.inv file also contains references for search.html, genindex.html, modindex.html, and py-modindex.html when I am pretty sure those pages don't exist for singlehtml builds (I know search.html doesn't).

References

• Incorrect objects.inv produced for singlehtml #10106
• Should singlehtml provide search utility? #3265
• singlehtml: Use same-document hyperlinks for internal project references #12551
• 🐛 Fix singlehtml target uris to be same-document references #11970