cmd/go: make 'go get' save sums for incidentally updated modules

When 'go get' updates a module, it may update another module in the
build list that provides a package in 'all' that wasn't loaded as part
of the 'go get' command. If 'go get' doesn't add a sum for that
module, builds may fail later.

With this change, 'go get' will fetch a sum for the content of an
updated module if we had a sum for the version before the update.

'go get' won't load the complete package graph, so there are still
cases where the build may be broken, like when an updated (but not
loaded) package imports a package from a new module.

Fixes #44129

Change-Id: I62eba3df4137a3e84e2ca8d549c36eec3670f08c
Reviewed-on: https://go-review.googlesource.com/c/go/+/322832


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