cmd/go: redirect vcs-test.golang.org repo URLs to a test-local server

cmd/go: redirect vcs-test.golang.org repo URLs to a test-local server

The new server reconstructs the vcs-test repos on the fly using
scripts that run the actual version-control binaries.

This allows those repos to be code-reviewed using our normal tools —
and, crucially, allows contributors to add new vcs-test contents
as part of a contributed CL.

It also prevents failures due to network errors reaching
vcs-test.golang.org (such as when developing offline), and allows us
to iterate on the repo contents without dealing with annoying and
unpredictable GCS caching behavior.

We can't quite turn down vcs-test.golang.org yet — this server doesn't
yet handle "go-import" metadata (and related authentication behaviors),
and doesn't serve Subversion repos.

But we're getting much closer!

For #27494.

Change-Id: I233fc718617aed287b0f7248bd8cfe1e5cebe96b
Reviewed-on: https://go-review.googlesource.com/c/go/+/421455
Run-TryBot: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Bryan Mills <bcmills@google.com>
Reviewed-by: Michael Matloob <matloob@golang.org>