cmd/go: in lazy modules, add transitive imports for 'go get' arguments

I needed to also update TestScript/mod_sumdb_golang.
It had been relying on 'go list -mod=mod' to add both the go.mod and
go.sum entries for the named package, but when 'go get' actually adds
all of the needed dependencies, lazy loading kicks in and 'go list'
doesn't end up needing the checksums for go.mod files.
We didn't detect the skew before because the 'go list' command was
(unexpectedly) also adding the missing dependencies, which triggered a
deep scan of the complete module graph.

For #45979

Change-Id: Ica917dee22c83ffa71c6ad0f2e189f911b73edf4
Reviewed-on: https://go-review.googlesource.com/c/go/+/328231


Trust: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Michael Matloob <matloob@golang.org>