cmd/go: move all Shell methods to shell.go
This is a pure code movement change.
Now that we've introduced a Shell type that lives in shell.go, move
all of its methods (which used to be on Builder) to shell.go as well.
Change-Id: Ie1802353c8a64590219b08893e8c74649228e5e5
Reviewed-on: https://go-review.googlesource.com/c/go/+/536536
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Austin Clements <austin@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>