hugolib: Fix automatic section pages not replaced by sites.complements

hugolib: Fix automatic section pages not replaced by sites.complements

When a home page or section page in language A is backed by a file and
language B has no content file for that section, the automatic page for
language B was not replaced by the complement from language A. This
happened because:

1. The content node shifter returned auto pages (not backed by files) as
   exact matches without trying complement fallback.
2. findContentNodeForSiteVector immediately returned auto pages as exact
   matches without considering file-backed complements.
3. getPagesInSection used Get (no fallback) for IncludeSelf, preventing
   complement resolution for home/section pages.

Fix by preferring file-backed complement pages over auto pages in the
shift and lookup mechanisms, and using fallback for self-inclusion.

Fixes #14540

Co-Authored-By: Joe Mooring <joe.mooring@veriphor.com>
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>