cmd/updatecontrib: add golang.org/{dl,x/mod,x/pkgsite} repos
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 <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Julie Qiu <8d32267b6b4884cf35adeaccde2b6857ae11aace@golang.org>
Reviewed-by: Carlos Amedee <ab5e2bca84933118bbc9d48ffaccce3bac4eeb64@golang.org>

golang/build,golang/build,golang/build,golang/build