cmd/go: move switch-only code from select.go to switch.go

cmd/go: move switch-only code from select.go to switch.go

Move NewerToolchain and related code from select.go to switch.go
because it is only used for the Switch operation, not for Select.
This is a separate CL containing only the code move, separate
from any other changes.

For #57001.

Change-Id: I41cf0629b41fd55c30a1e799d857c06039ee99b6
Reviewed-on: https://go-review.googlesource.com/c/go/+/500798
Reviewed-by: Michael Matloob <matloob@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
Auto-Submit: Russ Cox <rsc@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>