cmd/go: move module cache from $GOPATH/src/mod to $GOPATH/pkg/mod

Using $GOPATH/src/mod confuses too many tools.
$GOPATH/pkg/mod seems better for now.
It's also next to dep's cache, $GOPATH/pkg/dep.
If we do eliminate GOPATH/pkg for holding .a files (#4719)
then we could still keep it around for pkg/mod.
(Or we could move the module cache again then.)

Fixes #26401.
Fixes #26635.

Change-Id: I18f7da216ed9f490eded3c00d837fb086ae5b6a4
Reviewed-on: https://go-review.googlesource.com/126755


Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Rob Pike <r@golang.org>