This CL separates running shell commands and doing shell-like
operations out of the Builder type and into their own, new Shell type.
Shell is responsible for tracking output streams and the Action that's
running commands. Shells form a tree somewhat like Context, where new
Shells can be derived from a root shell to adjust their state.
The primary intent is to support "go build -json", where we need to
flow the current package ID down to the lowest level of command output
printing. Shell gives us a way to easily flow that context down.
However, this also puts a clear boundary around how we run commands,
removing this from the rather large Builder abstraction.