cmd/go: restore @latest behavior and support @upgrade in 'go get'

'go get path@latest' may now downgrade a module required at a
pre-release or pseudo-version newer than the latest released
version. This restores the 1.12 behavior and the ability to easily
roll back from a temporary development version.

'go get path@upgrade' is like @latest but will not downgrade.
If no version suffix is specified ('go get path'), @upgrade is
implied.

Fixes #32846

Change-Id: Ibec0628292ab1c484716a5add0950d7a7ee45f47
Reviewed-on: https://go-review.googlesource.com/c/go/+/184440


Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>