gddo-server: redirect /-/subrepo to pkg.go.dev/search?q=golang.org/x

Request to /-/subrepo are not redirected to
pkg.go.dev/search?q=golang.org/x instead of pkg.go.dev.

Change-Id: I582271b112a0ce482bedfd822523cf7e4dbc9059
Reviewed-on: https://go-review.googlesource.com/c/gddo/+/274697
Trust: Julie Qiu <julie@golang.org>
Run-TryBot: Julie Qiu <julie@golang.org>
Reviewed-by: Jonathan Amsterdam <jba@google.com>