x/pkgsite: flag packages that are no longer present in the latest version of the containing module

[bcmills]

What is the URL of the page with the issue?

https://pkg.go.dev/golang.org/x/tools/cmd/vet?tab=doc

What is your user agent?

Mozilla/5.0 (X11; CrOS x86_64 12607.82.0) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/79.0.3945.123 Safari/537.36 

Screenshot

What did you do?

Directly open the doc page for package golang.org/x/tools/cmd/vet.

What did you expect to see?

A prominent warning that the package in question no longer exists, ideally with similar style/coloration to the (currently-red) “Go to latest” link on the module overview page.

What did you see instead?

A normal documentation page for a very old version of golang.org/x/tools/cmd/vet, which was removed from the repo back in 2016, along with a cheerful blue “Latest” badge.

[bcmills]

(FYI @julieqiu @jba @findleyr)

[julieqiu]

Thanks for the feedback! We plan to fix this as part of an upcoming round of UX changes.

/cc @fflewddur