exp/types: move exp/types/staging -> exp/types

- removes exp/types/staging
- the only code change is in exp/gotype/gotype.go

R=iant
CC=golang-dev
https://golang.org/cl/6822068

src/pkg/exp/gotype/gotype.go

@@ -6,7 +6,7 @@ package main
 
 import (
 	"errors"
-	"exp/types/staging"
+	"exp/types"
 	"flag"
 	"fmt"
 	"go/ast"