x/pkgsite: broken links in directories section of pkg.go.dev/golang.org/dl

[julieqiu]

What is the URL of the page with the issue?

https://pkg.go.dev/golang.org/dl#section-directories

Screenshot

What did you do?

Click on go1.10 (or any link)

What did you expect to see?

A page for https://pkg.go.dev/golang.org/dl/go1.10, like https://godoc.org/golang.org/dl/go1.10.

What did you see instead?

400 bad request

[gopherbot]

Change https://golang.org/cl/278615 mentions this issue: internal/frontend: accept golang.org/dl/* paths