cmd/go: adjust pruning and switch toolchains if needed when go get changes go version

cmd/go: adjust pruning and switch toolchains if needed when go get changes go version

When we do 'go get', the Go version can change now.
That means we need to do the pruning conversions that
until now have only been necessary in go mod tidy -go=version.

We may also need to upgrade the toolchain in order to load enough o
the module graph to finish the edit, so we should let a TooNewError
bubble up to the caller instead of trying to downgrade the affected
module to avoid the error.

Revised from CL 498120.

For #57001.

Change-Id: Ic8994737eca4ed61ccc093a69e46f5a6caa8be87
Reviewed-on: https://go-review.googlesource.com/c/go/+/498267
Reviewed-by: Russ Cox <rsc@golang.org>
Auto-Submit: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>