cmd/go: only generate a go.mod file during 'go mod init'

In the general case, we do not know the correct module path for a new
module unless we have checked its VCS tags for a major version. If we
do not know the correct path, then we should not synthesize a go.mod
file automatically from it.

On the other hand, we don't want to run VCS commands in the working
directory without an explicit request by the user to do so: 'go mod
init' can reasonably invoke a VCS command, but 'go build' should not.

Therefore, we should only create a go.mod file during 'go mod init'.

This change removes the previous behavior of synthesizing a file
automatically, and instead suggests a command that the user can opt to
run explicitly.

Updates #29433
Updates #27009
Updates #30228

Change-Id: I8c4554969db17156e97428df220b129a4d361040
Reviewed-on: https://go-review.googlesource.com/c/162699


Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>