page.previous property in UI model should skip entries that are fragment of the previous item

The page.previous property in the UI model should skip past entries that are fragments of the previous item. Thus, instead of page.previous pointing to the last fragment in the previous page, it should point to the fragmentless page (or the first item for that page with a fragment if all items have fragments). This complements #622 (closed).

Here's an example to illustrate this logic:

* xref:quickstart.adoc[]
 ** xref:quickstart.adoc#install-nodejs[Install Node.js]
 ** xref:quickstart.adoc#install-antora[Install Antora]
 ** xref:quickstart.adoc#run-antora[Run Antora]
 ** xref:sample-projects.adoc[]

If the current page is sample-projects.adoc, the page.previous is quickstart.adoc instead of quickstart.adoc#run-antora. When computing the previous entry, subsequent entries that share the same URL without the hash part are not considered.