Make goto work with relative routes

[verschmelzen]

When using goto method from '@sapper/app' on relative paths, i.e. starting with ./, navigation is made relative to root.

I expect it to work as href in case when route is inside sapper application.

Now for every page that has relative navigation I have to implement prefetch and manualy store current path in component variable or reach for window.location.pathname.

I am using SSR mode and didn't test behavior in other cases.

[thgh]

Related to #984, my preferred workaround is using $page.path and $page.params however

[Conduitry]

Yeah I'd say this is a duplicate of #904.