cmd/go: get some.domain.net/... no longer works with custom domains #31241
% ~/go1.10/bin/go version
% ~/goroot/bin/go version
Observe that the first command does one Fetch per package in the expansion of go.starlark.net/... whereas the second fetches https://go.starlark.net/...?go-get=1, which doesn't exist.
This breaks the google3 update.go script for custom domains.