Show checkout step as a build command

[agjohnson]

Replace VCS code with standard BuildEnvironment execution. This will require rewriting a lot of the command execution in readthedocs.vcs_support. The goal of this change is to get the the checkout step output as standard build output, in the first pass using the LocalEnvironment. This will help surface errors with why a failure happened, now that we are removing this output from our verbose logging messages.