cmd/go: report error for empty GOPROXY list
    
    If GOPROXY is "", we set it to the default value,
    "https://proxy.golang.org,direct". However, if GOPROXY is a non-empty
    string that doesn't contain any URLs or keywords, we treat it as
    either "off" or "noproxy", which can lead to some strange errors.
    
    This change reports an error for this kind of GOPROXY value.
    
    For #39180
    
    Change-Id: If2e6e39d6f74c708e5ec8f90e9d4880e0e91894f
    Reviewed-on: https://go-review.googlesource.com/c/go/+/234857
    Run-TryBot: Jay Conrod <jayconrod@google.com>
    TryBot-Result: Gobot Gobot <gobot@golang.org>
    Reviewed-by: Bryan C. Mills <bcmills@google.com>
    Reviewed-by: Michael Matloob <matloob@golang.org>