cmd/go: add missing str imports to fix addmod and savedir

cmd/go: add missing str imports to fix addmod and savedir

The addmod.go and savedir.go testdata generation tools were missing the
"cmd/go/internal/str" import, causing them to fail to build. This change
adds the imports.

This makes progress on the work to use Join functions rather than path
manipulation using the path separator:
https://go-review.googlesource.com/c/go/+/463178

Change-Id: Ia70253afc0a351c646e672bd4178a8738e4e69cf
Reviewed-on: https://go-review.googlesource.com/c/go/+/524155
Run-TryBot: Bryan Mills <bcmills@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Bryan Mills <bcmills@google.com>