cmd/updatecontrib: add golang.org/{dl,x/mod,x/pkgsite} repos

These repositories use the Go license and have a minimal CONTRIBUTORS
file that points to the one in the main Go repository, so contributors
to those repositories should be considered by the updatecontrib tool.

For golang/go#12042.
Updates golang/go#36047.

Change-Id: Ia8932f37de0a58a8156f7857806a4d607ca3dd6d
Reviewed-on: https://go-review.googlesource.com/c/build/+/247777
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Julie Qiu <julie@golang.org>
Reviewed-by: Carlos Amedee <carlos@golang.org>

cmd/updatecontrib/updatecontrib.go

@@ -255,6 +255,10 @@ func file(name string) *acFile {
 // updated, and used to find contributors to add to the CONTRIBUTORS file.
 // It includes "go", which represents the main Go repository,
 // and an import path corresponding to each subrepository root.
+//
+// TODO(golang.org/issue/36047): Rewrite to use x/build/repos, being
+// mindful whether each repo is expected to contribute to the main
+// Go distribution's CONTRIBUTORS file or not.
 var repos = []string{
 	"go", // main repo
 	"golang.org/x/arch",
@@ -263,14 +267,17 @@ var repos = []string{
 	"golang.org/x/build",
 	"golang.org/x/crypto",
 	"golang.org/x/debug",
+	"golang.org/dl",
 	"golang.org/x/exp",
 	"github.com/golang/gddo", // The canonical import path for gddo is on GitHub.
 	"golang.org/x/image",
 	"golang.org/x/lint",
 	"golang.org/x/mobile",
+	"golang.org/x/mod",
 	"golang.org/x/net",
 	"golang.org/x/oauth2",
 	"golang.org/x/perf",
+	"golang.org/x/pkgsite",
 	"golang.org/x/playground",
 	"go.googlesource.com/proposal.git", // It doesn't have an /x/ vanity import path.
 	"golang.org/x/review",