cmd/go: suppress more errors in package-to-module loading

In CL 197059, I suppressed errors if the target package was already found.
However, that does not cover the case of passing a '/v2' module path to
'go get' when the module does not contain a package at its root.

This CL is a minimal fix for that case, intended to be backportable to 1.13.

(Longer term, I intend to rework the version-validation check to treat
all mismatched paths as ErrNotExist.)

Fixes #34746
Updates #34383

Change-Id: Ia963c2ea00fae424812b8f46a4d6c2c668252147
Reviewed-on: https://go-review.googlesource.com/c/go/+/199839


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