[vpython] Implement smart probing.

vpython/venv/venv_test.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 (
         "encoding/json"
         "flag"
         "fmt"
@@ skipping 29 matching lines @@
         return c
 }
 
 type resolvedInterpreter struct {
         py      *python.Interpreter
         version python.Version
 }
 
 func resolveFromPath(vers python.Version) *resolvedInterpreter {
         c := testContext()
-        py, err := python.Find(c, vers)
+        py, err := python.Find(c, vers, nil)
         if err != nil {
                 return nil
         }
         if err := filesystem.AbsPath(&py.Python); err != nil {
                 panic(err)
         }
 
         ri := resolvedInterpreter{
                 py: py,
         }
@@ skipping 218 matching lines @@
         path := actual.(string)
         switch _, err := os.Stat(path); {
         case err == nil:
                 return fmt.Sprintf("Path %q should not exist, but it does.", path)
         case os.IsNotExist(err):
                 return ""
         default:
                 return fmt.Sprintf("Couldn't check if %q exists: %s", path, err)
         }
 }