cmd/golangorg: redirect /x/ to pkgsite subrepo page

For golang/go#47281

Change-Id: I065e6cb1d301701304228a3e4f263f3e576f8cbe
Reviewed-on: https://go-review.googlesource.com/c/website/+/355809
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Jamal Carvalho <jamal@golang.org>
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Jamal Carvalho <jamal@golang.org>

cmd/golangorg/server.go

@@ -491,7 +491,7 @@ func loggingHandler(h http.Handler) http.Handler {
 func xHandler(w http.ResponseWriter, r *http.Request) {
 	if !strings.HasPrefix(r.URL.Path, "/x/") {
 		// Shouldn't happen if handler is registered correctly.
-		http.Redirect(w, r, "https://pkg.go.dev/search?q=golang.org/x", http.StatusTemporaryRedirect)
+		http.Redirect(w, r, "https://pkg.go.dev/golang.org/x", http.StatusTemporaryRedirect)
 		return
 	}
 	proj, suffix := strings.TrimPrefix(r.URL.Path, "/x/"), ""
@@ -499,7 +499,7 @@ func xHandler(w http.ResponseWriter, r *http.Request) {
 		proj, suffix = proj[:i], proj[i:]
 	}
 	if proj == "" {
-		http.Redirect(w, r, "https://pkg.go.dev/search?q=golang.org/x", http.StatusTemporaryRedirect)
+		http.Redirect(w, r, "https://pkg.go.dev/golang.org/x", http.StatusTemporaryRedirect)
 		return
 	}
 	repo, ok := repos.ByGerritProject[proj]

cmd/golangorg/testdata/x.txt

@@ -19,7 +19,7 @@ code == 404
 
 GET https://golang.org/x/
 code == 307
-header location == https://pkg.go.dev/search?q=golang.org/x
+header location == https://pkg.go.dev/golang.org/x
 
 GET https://golang.org/x/In%20Valid,X
 code == 404