cmd/go/internal/modload: reject empty go.mod files

cmd/go/internal/modload: reject empty go.mod files

Don't add a module declaration to a go.mod file when
loading a module. Require a user to call go mod init or to
add the module declaration themselves.

Fixes #35070

Change-Id: If5543580d3c1cfee1fc528eb853b872c4173ca82
Reviewed-on: https://go-review.googlesource.com/c/go/+/234107
Run-TryBot: Michael Matloob <matloob@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>