…” error

If we have found a repository at the requested version but it does not
contain a go.mod file in an appropriate subdirectory, then the module
with the given path does not exist at that version. Therefore, we
should report it with an error equivalent to os.ErrNotExist so that
modload.Query will continue to check other possible module paths.

Updates #27173

Change-Id: Ica73f4bb97f58e611a7f7d38183ee52fef5ee69a
Reviewed-on: https://go-review.googlesource.com/c/go/+/183618
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>