Sever isolate package's dependency on client/internal.

client/isolate/utils.go

Index: client/isolate/utils.go
diff --git a/client/isolate/utils.go b/client/isolate/utils.go
index 2c1fdda2d5fcfe5ce656bd21509462fde947a47b..b66ab49e66264eaa6e71b03458d83b9fef666558 100644
--- a/client/isolate/utils.go
+++ b/client/isolate/utils.go
@@ -8,6 +8,7 @@ import (
 	"fmt"
 	"log"
 	"path"
+	"runtime"
 	"strings"
 
 	"github.com/luci/luci-go/common/errors"
@@ -133,3 +134,8 @@ func posixRel(basepath, targpath string) (string, error) {
 	}
 	return targ[t0:], nil
 }
+
+// IsWindows returns True when running on the best OS there is.
+func IsWindows() bool {
+	return runtime.GOOS == "windows"
+}