cmd/go: only add a 'go' directive to the main module when the go.mod file will be written

Then, write the 'go.mod' file with that version before further
processing. That way, if the command errors out due to a change in
behavior, the reason for the change in behavior will be visible in the
file diffs.

If the 'go.mod' file cannot be written (due to -mod=readonly or
-mod=vendor), assume Go 1.11 instead of the current Go release.
(cmd/go has added 'go' directives automatically, including in 'go mod
init', since Go 1.12.)

For #44976

Change-Id: If9d4af557366f134f40ce4c5638688ba3bab8380
Reviewed-on: https://go-review.googlesource.com/c/go/+/302051


Trust: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>
Reviewed-by: Michael Matloob <matloob@golang.org>