cmd/go: adjust pruning and switch toolchain when needed in 'go mod tidy'

cmd/go: adjust pruning and switch toolchain when needed in 'go mod tidy'

'go mod tidy' may resolve an imported package by added a dependency
that requires a higher 'go' version, which may activate graph pruning
(if the version goes from below go 1.16 to above it), and may even
require switching to a newer toolchain (if the version is not
supported by the current one).

For #57001.

Change-Id: Ic8e9b87d5979b3a6d1ee70f1f2bf2eea46b1bb0d
Reviewed-on: https://go-review.googlesource.com/c/go/+/499676
Reviewed-by: Russ Cox <rsc@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Bryan Mills <bcmills@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>