Move third_party code under third_party/src so it can be used in $GOPATH.

third_party/src/bitbucket.org/kardianos/osext/osext_plan9.go

@@ -0,0 +1,20 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package osext
+
+import (
+    "syscall"
+    "os"
+    "strconv"
+)
+
+func executable() (string, error) {
+    f, err := os.Open("/proc/" + strconv.Itoa(os.Getpid()) + "/text")
+    if err != nil {
+        return "", err
+    }
+    defer f.Close()
+    return syscall.Fd2path(int(f.Fd()))
+}