Parser::replaceInternalLinks: fix batching

The parser unnecessarily made individual checks for existence of
pages that were neither in LinkCache nor linked only with a fragment.
A Title::isKnown() call in Parser::replaceInternalLinks2() (added in
bca8b8ad7d2f) caused this.

Title::isKnown() was used to avoid treating a link to a distinct page
as a self-link even when the title happened to match one of the variants
returned by Language::autoConvertToAllVariants(). This change fixes
the bug by moving the problematic portion of the self-link check into
LinkHolderArray::doVariants().

Change-Id: I586e11e8b47308980ea04087ebc4246c397a8f53