cmd/go: add support for 'go' and 'toolchain' repos in modfetch

cmd/go: add support for 'go' and 'toolchain' repos in modfetch

To make the new go lines work with 'go get' as minimum requirements,
this CL creates a synthetic 'go' module that has as its versions the valid
versions that can be listed on the 'go' line.

In preparation for allowing 'toolchain' changes as well, an equivalent
synthetic module is introduced for 'toolchain'.

For #57001.

Change-Id: Id0ebbd283f0f991859d516d21dffe59a834db540
Reviewed-on: https://go-review.googlesource.com/c/go/+/497080
Reviewed-by: Bryan Mills <bcmills@google.com>
Run-TryBot: Russ Cox <rsc@golang.org>
Auto-Submit: Russ Cox <rsc@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>