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 <[email protected]>
Auto-Submit: Austin Clements <[email protected]>
Reviewed-by: Bryan Mills <[email protected]>

Author: aclements