crostestutils: fix import path and add go.mod

Switching to Go modules mode, which is the default dep
management mode in newer Go versions, requires that all
non standard-library import paths be prefixed with an URL,
so imports like the following will not work anymore:

import "firmware/internal/pkg/dutio"

This is due to the the fact that direct import paths are
now reserved strictly for the Go standard library pkgs
and all non-std pkgs are required to have an URL prefix.

A symlink with the URL path is added to work in GOPATH
mode. It will be dropped when switching to modules mode.

While at it, we also add a go.mod definition which
explicitely lists the dependencies required by the tools.

BUG=b:187795288
TEST=Local builds; CQ.

Cq-Depend: chromium:3666724
Change-Id: I3e993871c6b678fca9928953ba027bc5f0187e7e
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crostestutils/+/3574201
Auto-Submit: Adrian Ratiu <adrian.ratiu@collabora.corp-partner.google.com>
Commit-Queue: Adrian Ratiu <adrian.ratiu@collabora.corp-partner.google.com>
Tested-by: Adrian Ratiu <adrian.ratiu@collabora.corp-partner.google.com>
Reviewed-by: Kevin Shelton <kmshelton@chromium.org>
4 files changed