vpython: Add CIPD support.

vpython/README.md

 ## vpython - simple and easy VirtualEnv Python
 
 `vpython` is a tool, written in Go, which enables the simple and easy invocation
 of Python code in [VirtualEnv](https://virtualenv.pypa.io/en/stable/)
 environments.
 
 `vpython` is a simple Python bootstrap which (almost) transparently wraps a
 Python interpreter invocation to run in a tailored VirtualEnv environment. The
 environment is expressed by a script-specific configuration file. This allows
 each Python script to trivially express its own package-level dependencies and
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 An enviornment specification file is a text protobuf defined as `Spec`
 [here](./api/env/spec.proto). An example is:
 
 ```
 # Any 2.7 interpreter will do.
 python_version: "2.7"
 
 # Include "numpy" for the current architecture.
 wheel {
-  path: "infra/python/wheels/numpy/${platform}-${arch}"
+  name: "infra/python/wheels/numpy/${platform}-${arch}"
   version: "version:1.11.0"
 }
 
 # Include "coverage" for the current architecture.
 wheel {
-  path: "infra/python/wheels/coverage/${platform}-${arch}"
+  name: "infra/python/wheels/coverage/${platform}-${arch}"
   version: "version:4.1"
 }
 ```
 
 This specification can be supplied in one of three ways:
 
 * Explicitly, as a command-line option to `vpython` (`-spec`).
 * Implicitly, as a file alongside your entry point. For example, if you are
   running `test_runner.py`, `vpython` will look for `test_runner.py.vpython`
   next to it and load the environment from there.
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 #### Shebang (POSIX)
 
 If your script uses implicit specification (file or inline), replacing `python`
 with `vpython` in your shebang line will automatically work.
 
 ```sh
 #!/usr/bin/env vpython
 ```