gowalker does not properly target package-relative images from md files

[kortschak]

Navigate to https://gowalker.org/github.com/ev3go/ev3 then look at the alt-text "Gopherbrick". The link it contains is https://gowalker.org/github.com/ev3go/gopherbrick.png which does not exist. In another situation where the link is not package-relative, the correct link is used and the image renders (e.g. https://gowalker.org/github.com/biogo/biogo).