x/pkgsite: multi-module projects have confusing versions

[julieqiu]

https://pkg.go.dev/cloud.google.com/go@v0.46.2/storage says cloud.google.com/go/storage v0.46.2 is the latest.

https://pkg.go.dev/cloud.google.com/go@v0.46.1/storage has a Go to latest badge which will take the user to v0.46.2.

But, cloud.google.com/go/storage was carved into its own module after v0.46.2. So, if you go straight to https://pkg.go.dev/cloud.google.com/go/storage (which currently shows https://pkg.go.dev/cloud.google.com/go/storage@v1.10.0), you will get

[bcmills]

#37631 is closely related.