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 <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
Reviewed-by: Bryan Mills <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Auto-Submit: Bryan Mills <[email protected]>

Author: dagood

src/cmd/go/testdata/addmod.go

@@ -20,6 +20,7 @@ package main
 
 import (
 	"bytes"
+	"cmd/go/internal/str"
 	"flag"
 	"fmt"
 	"internal/txtar"

src/cmd/go/testdata/savedir.go

@@ -15,6 +15,7 @@
 package main
 
 import (
+	"cmd/go/internal/str"
 	"flag"
 	"fmt"
 	"internal/txtar"