vpython: Add environment spec package.

vpython/api/env/spec.proto

-// 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.
-
-syntax = "proto3";
-
-package env;
-
-
-message Spec {
-  // The Python version to use. This should be of the form:
-  // "Major[.Minor[.Patch]]"
-  //
-  // If specified,
-  // - The Major version will be enforced absolutely. Python 3 will not be
-  //   preferred over Python 2 because '3' is greater than '2'.
-  // - The remaining versions, if specified, will be regarded as *minimum*
-  //   versions. In other words, if "2.7.4" is specified and the system has
-  //   "2.7.12", that will suffice. Similarly, "2.6" would accept a "2.7"
-  //   interpreter.
-  //
-  // If empty, the default Python interpreter ("python") will be used.
-  string python_version = 1;
-
-  // A definition for a remote package. The type of package depends on the
-  // configured package resolver.
-  message Package {
-    // The path of the package.
-    //
-    // - For CIPD, this is the package name.
-    string path = 1;
-
-    // The package version.
-    //
-    // - For CIPD, this will be any recognized CIPD version (i.e., ID, tag, or
-    //   ref).
-    string version = 2;
-  }
-  repeated Package wheel = 2;
-
-  // The VirtualEnv package.
-  //
-  // This should be left empty to use the `vpython` default package
-  // (recommended).
-  Package virtualenv = 3;
-}