regexp: move to old/regexp, replace with exp/regexp

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/5127042
diff --git a/src/pkg/index/suffixarray/suffixarray.go b/src/pkg/index/suffixarray/suffixarray.go
index cff7daa..4747ba9 100644
--- a/src/pkg/index/suffixarray/suffixarray.go
+++ b/src/pkg/index/suffixarray/suffixarray.go
@@ -18,10 +18,10 @@
 
 import (
 	"bytes"
-	"exp/regexp"
 	"gob"
 	"io"
 	"os"
+	"regexp"
 	"sort"
 )