Add a way to list packages by prefix

[broady]

https://godoc.org/-/subrepo has a special index to display all the golang.org/x/... subrepos.

It would be nice to have this for any package path prefix. For example, to display all of the google.golang.org/... repos.

[dmitshur]

That does sound nice. Some queries of that nature may return an exorbitant amount of packages without pagination. Still, it will be less than the page that displays all packages known to godoc.org.

@broady, in addition to google.golang.org/..., would this feature request would also include queries like honnef.co/go/..., github.com/username/... and github.com/...?

[broady]

Exactly. We should paginate. That other page that lists all packages should be paginated, anyway!