cmd/go: assume Go 1.16 semantics uniformly for unversioned modules

However, still only trigger -mod=vendor automatically (and only apply
the more stringent Go 1.14 vendor consistency checks) if the 'go'
version is explicit. This provides maximal compatibility with Go 1.16
and earlier: Go 1.11 modules will continue not to fail vendor
consistency checks, but scripts that assume semantics up to Go 1.16
for unversioned modules will continue to work unmodified.

Fixes #44976
For #36460

Change-Id: Idb05ca320023f57249c71fc8079218e8991d1ff9
Reviewed-on: https://go-review.googlesource.com/c/go/+/308509


Trust: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Jay Conrod <jayconrod@google.com>