cmd/go: introduce Shell abstraction

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.

For #62067.

Change-Id: Ia9ab2a2d7cac0269ca627bbb316dbd9610bcda44
Reviewed-on: https://go-review.googlesource.com/c/go/+/535016
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
Auto-Submit: Austin Clements <austin@google.com>

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