title: How to create virtualenv with bazel 0.24, then use that in py_runtime
body: <p>I have an existing project that uses bazel 0.24. The project does not define a <code>py_runtime</code>. Right now, <code>py_test</code> targets can import items from <code>/usr/local/lib/python2.7/dist-packages/</code> which breaks encapsulation and can cause version drift from what's defined in <code>requirements.txt</code>.</p>
  
  <p>I cannot upgrade my bazel version at this time, so the toolchains work that's going on is not available to me.</p>
  
  <p>I would like to define a bazel rule to build a virtual environment at <code>.venv/py27/...</code> using the <code>requirements.txt</code> file I have, so that I can use that environment in a <code>py_runtime</code>. I'll be doing the same with python 3.6 as well.</p>
  
  <p>How can I express the creation of the virtual environment as a bazel target? Are there any special considerations for doing so and then using that as the runtime? How should the <code>py_runtime</code> be defined?</p>