vpython: Add VirtualEnv creation package.

vpython/venv/system_posix.go

Index: vpython/venv/system_posix.go
diff --git a/vpython/venv/system_posix.go b/vpython/venv/system_posix.go
new file mode 100644
index 0000000000000000000000000000000000000000..a7ff9169492d57ab5918132413a17f955f5f3c11
--- /dev/null
+++ b/vpython/venv/system_posix.go
@@ -0,0 +1,16 @@
+// Copyright 2017 The LUCI Authors. All rights reserved.
+// Use of this source code is governed under the Apache License, Version 2.0
+// that can be found in the LICENSE file.
+
+// +build darwin linux freebsd netbsd openbsd android
+
+package venv
+
+import (
+	"path/filepath"
+)
+
+// venvBinPath resolves the path to a VirtualEnv binary.
+func venvBinPath(root, name string) string {
+	return filepath.Join(root, "bin", name)
+}