vpython: Add VirtualEnv creation package.

vpython/venv/system_windows.go

+// 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.
+
+package venv
+
+import (
+        "path/filepath"
+)
+
+// venvBinPath resolves the path to a VirtualEnv binary.
+func venvBinPath(root, name string) string {
+        return filepath.Join(root, "Scripts", name)

[iannucci, 2017/02/23 00:54:21]

wdyt about just having a `const BinDir = "Scripts"`? Doesn't seem like it needs
a whole function. In fact, you could probably just pick it in an init() method
to avoid having 2 extra files. Just a thought.

[dnj, 2017/02/23 20:38:49]

Meh, build-time is nicer, and I think this puts the constant into context
better.

+}