The golang.org/dl repo is a special case because it doesn't include
the "x" element like other golang.org/x repos. Handle it specially.

This functionality is factored out of the gido website
so that it can be reused across a wider set of projects.

Follows https://dmitri.shuralyov.com/website/gido$commit/9ec8098d1ae7dcc4e4bcc249640bb5dec2a42b2c.