QtHelp: Make the namespace fallback for a file deterministic

When no namespace with the requested name or version contained the
file, namespaceForFile() returned the first match, so the order of
rows in the collection file decided which documentation was shown.

Return the match with the highest version instead, preferring the
first name in alphabetical order when versions are equal.

Task-number: QTBUG-150688
Pick-to: 6.12
Change-Id: I4c17b996f6d62ccbb19aa3ffe647364709c3a32f
Reviewed-by: Kai Köhne <kai.koehne@qt.io>