cmd/go: make go line declare minimum required Go version

For #57001, enforce the go line as declaring the minimum required
version of Go that can compile a module.

Modules that maintain compatibility with old versions of Go
but want to make use of new features in //go:build-constrained files
will be able to do so: the //go:build constraint will be interpreted
as changing the minimum Go version for that file and will unlock
the Go features allowed in that version.

Change-Id: Ibeeb7c93ce7ea2e5187d78af0757cbfac19484a0
Reviewed-on: https://go-review.googlesource.com/c/go/+/476279
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
Run-TryBot: Russ Cox <rsc@golang.org>
Auto-Submit: Russ Cox <rsc@golang.org>

0 files changed, 0 insertions, 0 deletions