Use looks_like_pbl() to disallow .pbl file inclusion (bug #23504)

This function is implemented using case-insensitive pattern matching,
unlike filesystem::ends_with(). I missed this when writing my original
fix, so the vulnerability still applied to .pbl files on a
case-insensitive filesystem (e.g. NTFS and FAT* on Windows) by using
different case to bypass the check.