MWLinkTargetInputWidget: Normalise value before matching

Don't suggest a "new page" (redlink) for values that are
strictly different but the same as a suggestion according
to mw.Title (e.g. input "foo" with an existing "Foo" page
should not suggest a redlink to inexistant page "foo").

Bug: 48476
Change-Id: I66f5fc56554984af58d6223dc6cd76b3ab9940bd